Jan 13, 2021 - 03:41 AM
One idea that I do have is that most android phones have test modes that allow you to see signal strength and other more involved measurements.
Field Test Mode: What it is and How To Enable it on Your Phone (ubersignal.com)
Test mode will work even if the phone isn't turned on for the network- but I suspect it might require a SIM card in the phone to work. So if you purchase old android phones for that carrier (with SIM card - even if not active) you should be able to test signal strength. (I suspect this is accurate- but don't know for sure)