GSM module? I think it costs several dollars. They are already installed into electricity meters for example.

The lifetime subscription :)

It doesn't need to be lifetime, only as long as devices are supported. Also, electricity meters makers somehow solved this problem. I am sure a subscription won't be expensive if the device doesn't use too much data. Also the device maker can pay with collected data or advertisement.

Electricity meters are already conveniently plugged to a wire?