Uninstal the app, reboot your phone, re-download and install the app, choosing to connect to existing system.

It may then ask you to do a system update, which you should do.

FYI… For full functionality of the Sonos S2 app, iOS 14 and higher is required. For the S1 app, iOS 12 and higher is required. Read more here:

https://support.sonos.com/en-us/article/sonos-app-requirements

Also be sure to turn off all auto-update options, on phone and speakers. 

If you are using S2 you no longer confirm to the Sonos system requirements. The system and the app need updates for security reasons, new possibilities for the systens (like SVC) and to eliminate software bugs. The phone app and the system need to be on the same page so when the system has an update your phone app will need one too. Sonos does not immediately stop support for apps on older OS's but gradually limits what you can do with it. Turning off updates could make your system a security risk.

By the way: my bank app also needs at least iOS 14.

The alarm is set up in the system, not the app. The app is just a remote to the system.

So, this alarm will go on unless I reset the speaker?

I guess I will only use it as a Airplay speaker and get rid of Sonos apps...

You could borrow a more recent phone to end the alarm.

If you have no need to use the options the Sonos app gives you (Trueplay, to add speakers or set up a new system) using Airplay only is a good option.