Often a power-off reset helps cure issues. Have you tried that with your phone? Also, are you on the current opsys?

The fact that her phone is ok (as is my XR with S2, and so are many others!) suggests its a specific setting on your phone, somewhere. 

I suggest uninstalling any SONOS Apps from your phone, then reboot the phone as suggested. Check for phone updates, then reinstall the SONOS App(s). As you install the SONOS App(s) make sure to indicate that you are working with an existing system. Don’t setup a new system by accident.

If you can’t even install the S2 app on your phone then your phone has serious problems, unrelated to Sonos. you might need to back up the phone, wipe it, then reload the backup to get the phone into a working state.

I have a similar problem. New Sonos app installed on my up to date iPhone XR and iPad. Sometimes it connects, sometimes it doesn’t. Or it connects then disconnects two minutes later. My husband has a similar issue on an older iPhone. Are you recommending uninstalling the new Sonos app and then starting from zero? (We have 2 internet networks: husband’s phone will connect to newer one, my iPhone will only connect to older network—when it connects at all.) Why is it so unstable? It used to be so simple!

Note that the controller is essentially only a remote control. The actual processing is done on the speakers themselves, so there is no risk in uninstalling the app from your phone, rebooting the phone, and installing a fresh copy, if you the ‘connect to existing system’. 

If I were to hazard a guess as to why it’s so unstable, I suspect some corrupt memory in the phone, which a reboot should clear up. 

That isn’t remotely similar: the OP has trouble actually installing the apps, which is an iOS issue. Your connectivity problem is unrelated. If you would like help with your issue, I suggest opening a fresh thread.