Not sure what you’re really asking here. Are you wanting to set up a WiFi connection in addition to a wired (SonosNet) connection?
 

You’d need to add the hotspot WiFi SSID to the Sonos controller, and then turn on the hotspot when it’s needed, the Sonos should just switch, as far as I’m aware. However, if a decvice is being used as a hotspot, it can not be used as a controller, so you’d need another device connected to the hotspot to use as a controller for your Sonos system. 

Thanks. 
 

I basically need to be able to toggle between my 5g hotspot on my phone and the sonosnet which is networked by Ethernet directly to our router/modem. 
 

I’ll use the Ethernet 99 percent of the time unless the internet goes out. Then I’d like to easily switch to the hotpot. 
 

This would have to be an easy and quick switch on the Sonos app since will, if needed, happen during events. 

When your Internet connection goes down, you would need to break the network connection to the router or power down the router. Otherwise SonosNet will wait for WAN service. At that point if you had entered the hotspot SSID and password, the SONOS units will switch to the hotspot. None of the SONOS units should be wired to each other or a network switch.

There will be some mischief when it’s time to renew DHCP addresses because the router has changed without rebooting the SONOS units. There will be more mischief when the connection is switched back to the regular router.

I think that a better plan would be to connect the router’s WAN connection to the hotspot. But, there is still an issue here because you may need to reboot both the router and modem when you transfer in each direction. Of course, if you are using an all in one Gateway it may not be possible to connect it to the hotspot.

There are routers, normally used in offices, designed for dual Internet connections. These will fallback to an alternate connection if the primary goes down. You would only need to power up the hotspot.

If you go with the “hook the router to the internet or phone” option:

As said IP addressing may well be an issue, more likely for the IP v6 links than the v4 ones. The v4 IP addresses are almost always local cRFC1918] ones that won’t change with the WAN’s connection while some of the v6, but not always all, will change. Having static/reserved IP v4 addresses should minimize possible DHCP v4 problems.

Depending on the router’s features it may automatically see the changes in the WAN link and update the internal settings or it may need a poke to do that. Some routers offer a renew the WAN DHCP lease option, others may require a reboot. Testing ahead of need will tell you where your router falls.

I’ve looked at several dual WAN routers and aside from the expense I haven’t found one that is happy with the usual home Internet connection that uses IPv6 prefix-delegation. Again, that doesn’t impact v4 RFC1918 addresses but does the v6 ones. Test and hope for the best but be prepared for some deep networking research if it doesn’t just work.