More than a room away the bluetooth will not work and your watch needs to connect to the phone via WiFi or mobile network which means both the watch and phone have to be connected to a network.
I'm having a similar problem. I bought a new A31, and my Galaxy Fit2 (that worked well with my J6) when I disconnect once (because I came out of range or when I go to sleep ), afterwards it starts to reconnect and disconnect. I have to disconnect it completely from the phone and reconnect as the initial setup. I have the last firmware for the watch.