Firmware 3.5.3 has just been released into beta. It stabilizes the WiFi connection in most of our testing setups significantly. Please give it a go and report back in this thread any experiences.
It might not solve the problems with some Unifi setups where the Smart Lock does not reconnect to Wifi anymore after some time. If you are affected and your Smart Lock does not automatically reconnect, please try to access it via BLE, unlock the door via button or manually by turning the knob on the Smart Lock and check if it comes back online after it (All 3 things will initiate an immediate WiFi reconnect attempt).