Apple has removed the ability to use the hardware buttons with a ‘remote control app’, and have restricted it to only work with apps that generate the sound on the Apple device itself. 

You could use AirPlay 2 to send a signal to your Beam, but you would be using another music app, not the Sonos app, and then your hardware buttons would function. 

If it were me, I’d address ‘the buggy’ part of your system, it’s most likely either an issue with wifi interference , or a duplicate IP address issue. 

Thank you for the response. App is not just slow but partly unusable as we need to be precise to adjust volume slider. Changing volume should be easy and not a task. To me, it feels that Sonos has dropped the ball on usability. No wonder they will not see the sales they hope for if they continue to leave bad taste after using their products