Why is it so difficult to have 2 Era 300 working independently in the same network?

I’m struggling to get an apparently simple setup at home. I have 2 sonos era 300 in two separate rooms and had the candid idea to have them registered under the same account.

The only complexity my network has is a simple wifi repeater upstairs. The repeater replicates the network through LAN. 

The result is quite frustrating, as the app is only able to recognize 1 system at a time and momentarily forgets the existence of the other one. Switching from one to the other is virtually impossible. Sometimes reconnecting the smartphone to the network works after multiple attempts. If the Sonos app is in a particularly nasty mood, it will decide to pick exactly the opposite device and stick with it no matter what.  Spotify seems able to differentiate devices and control them… to a certain degree, as last updates form Sonos seem to have limited this possibility.

I’m quite surprised this is not plug & play as this is basic smart home stuff. Tried to fix it for some time when I was hooked by the quality of those speakers, but after a while this is just a quite annoying feature (or lack of feature). I tried also to do it also through Alexa, the result was even more chaotic and horrible (what a surprise).

In some help threads that I can’t find again I read about solutions like having two separate smartphone devices dedicated to control individually each Era 300, but that seems quite raw for a technical solution.

I would be very grateful to anybody giving me some hints.