I know this is a bit of an old topic, but the issue still exists for me. Hoping to add some more data points here and find out if anyone has found the magic combination of settings.
I have Grandstream Wave on three smart devices registered to an UCM6102. The android stays reliably connected but only if I leave the registration notification turned on. The ipad is somewhat better, but still disconnects periodically. The iphone will looses it’s registration when locked for a period of time (still narrowing that down). It eventually re-registers when the phone is unlocked or you open the GS Wave app.
I have keep alive turned on and set for 20 seconds. I’ve played around with the registration duration but went back to the default of 60 minutes on my android and it stays connected, but not the iphone. One difference between the android and iphone app is there is no “Enable Session Expiration” on/off button for the iphone. It is set to off for the android.
I have notifications turned on as well as the “Background App Refresh” set to on for GS Wave on the ios devices.