It’s hard to tell whether or not this is a Sonos issue, or an Apple device issue, or even a network issue. 

I’d try several things. First, I’d take this device to where you have it set up on WiFi, and update it. I’m not sure why you can’t do that currently, my system is currently working off of ‘mobile’ network provided by T-Mobile. It has been working fine (with occasional, but very infrequent dropouts) since I moved to this home, several years ago.

In any case, I’d also try making sure my Apple devices were up to date, and the Sonos was within range. Bluetooth claims 15 feet, if memory serves, I’d stick to much smaller distances, say 9 feet (3 meters, give or take) to ensure the Sonos is getting good ‘signal’. 

I’d look at the wifi interference FAQ. While Bluetooth isn’t WiFi, it is a radio signal, and much of that information could be applicable. Bluetooth is subject to much of the same physics as WiFi, it isn’t ‘magic’. 

Finally, I’d likely call Sonos Support to discuss it.

When you speak directly to the Support staff, they have tools at their disposal that will allow them to give you advice specific to your network and Sonos system. They might have some alternate suggestions that might assist