Newbie help diagnosing sensor dropping off network

It all depends where are you located - US and EU/AU Zigbee device availability is different...

This is the right approach!
You need to build a strong Zigbee mesh, by adding mains-powered zigbee devices - always-on plugs/sockets as an example, but never Zigbee bulbs that may be turned off.

You can also check this thread :

but the solution there was more of a complex workaround...