This is what routers are actually for. We use them in our setups simply because it's an easy way of providing DHCP rather than setting up a network server or running everything with fixed IP (which would be an option but a real pain in the ... to manage), but their intended use is to separate out local network traffic and internet bound traffic, providing a doorway between your LAN and the internet (WAN).

The simple option is to run a cable from your routers WAN port to the house LAN (if available). You'd need to do some minor re-config to ensure your LAN addressing scheme is different to the house LAN addressing else your NAT will complain.

The harder way is to run a cable from the WAN socket to another laptop/pc that has your iPhone connected and create a network Bridge. You can't use an existing laptop on the LAN side of your router for this. This is advance level stuff though.

Put simply, a personal hotspot is intended as just that... personal.

I have read your question with much interest. If we cut to the chase it appears your main objective is to have the ability to stream your iPhone running spotify or other audio streaming service to your Apple Express while it is running DHCP to all connected devices and allowing iPad control. My rig was not too different being a CS/RM with AVB switch and a Apple Extreme connected to the Ethernet port of the AVB switch running in DHCP mode. I have an Express as well and as far as I can recall it won't do both jobs at the same time.
The idea of running two networks together is fraught with problems. It impacts your remote control signal and your AVB stream integrity which as you have said are important in your set up. It is extremely important control devices only see a single connection point as it can cause massive confusion and drop outs. To this end it is recommended you shut down wifi on any computers and cat5e or cat6 cable where ever possible.
I can't see that static addressing will help with your iPhone streaming but it works fine in a closed network and having said that so does DHCP. Obviously you can stream spotify to your phone from your ISP and I would suggest you could use an adaptor to give you a hard wired output that could be used as an input into the CS18 or RM for BGM, the Tape in on the RM is handy for this. Alternatively you could use a small Bluetooth receiver if you want a bit less cabling. AVB will transmit either of these to your RM from the CS if you want to locate it there for playback.There may be another reason you want to do the wireless streaming to the Express but you haven't indicated what that is.
As far as updating your devices from the internet you could use your iPhone hotspot to do that but I would suggest you do that pre or post gig and not during. Having your hotspot on during a show will compromise your control connectivity which is a game changer in my opinion.
Others here will have more advanced network knowledge which may offer another option.
I hope this is of some help?

Really good advice here, I have to say. I should have known all this when I started getting involved in this complex technology. It's a miracle that I got everything working...

Let me add two remarks:

1) If RM and Mac mini are connected via FireWire, the Mac mini doesn't need a wired AVB connection. It only needs to join the WiFi network, which automatically registers Capture and Studio One in the UCNET menue of the CS18AI and all mobile devices involved. Capture becomes part of UC Surface, for Studio One you can use "Studio One Remote" (free download).

2) The router needs to be connected to port L (or E) of the Motu AVB switch, not to the Mac mini.

My RM/Computer rig network stay offline most all of the time. When I need to update the computer or RM mixer from the internet I just roll the rack in my house and plug the WAN from my rack router into the LAN of my home router with ethernet cable and BAM I have internet and can update eveything. I do this about once every two years or so. . . when they put out a good update I want. . .

Steve

ScottProAudio nailed it. Turn off the iPhone hotspot and hardwire the audio output to your tape in on the RM or Channels in on the CS.

I use nearly the identical setup live. I ALWAYS hardwire my audio playback source whether it is another laptop, iPhone, iPod, or MP3 player. The only thing I do wirelessly is WiFi for iPad control of UC, and this I do only on a 'hidden' network that other devices cannot see or ping. Wireless connections are less reliable than wired connections, and I like to put forth a very professional presentation, so I avoid it at all costs. That is actually why I have a CS18, so I can go hands on, in case of a wireless connectivity issue with the iPad. I also have a second iPad, and UC running on my laptop along with Capture. Over the years, I have had cause to use everyone of these backup measures to put on a seamless show. The performers and audience never knew of any issues.

To update my AI devices, I copy the update files to freshly reformatted SanDisc USB drives and update from USB. I have one USB drive for each different type of AI device i have: RM16, RM32, CS18, 328, and 18s. Those USB drives have copies of the old drivers in folders, and the current drivers in the root. I have never had to use them in a live scenario emergency.

ScottProAudio nailed it. Turn off the iPhone hotspot and hardwire the audio output to your tape in on the RM or Channels in on the CS.

Y'all are missing the point..

6. I would like to be able to transmit Spotify or iTunes music from the hotspot or idevice to an airport express on stage via the apple network.

There are dozens of ways of getting playback into the system, but he wants internet access to stream music..

Karyn wrote

ScottProAudio nailed it. Turn off the iPhone hotspot and hardwire the audio output to your tape in on the RM or Channels in on the CS.

Y'all are missing the point..

6. I would like to be able to transmit Spotify or iTunes music from the hotspot or idevice to an airport express on stage via the apple network.

There are dozens of ways of getting playback into the system, but he wants internet access to stream music..

I get the point completely. My recommendation is to NOT do it that way, and pick a more reliable alternative.

Sorry for the delay in getting back to you, I've been traveling.

I should like to thank you all for the excellent feedback which I'm working through.

Yes, I know that a personal hotspot is personal but what do you do when your in a remote spot and need a download ? You do what you can.

That said I do try to keep my system as free of internet updating as possible to ensure stability.

Not needing the cat cable to the router when FW connected I have not tried. Thanks for the tip. One less connection the better.

I'm setting up the system for some trials during the holidays - I get back if I have any interesting discoveries.

Thanks again for your constructive support !