I would put a thermistor of known type onto the bed and into the nozzle. Or at least check how you have the Configuration.h file setup and make sure what it says for the Thermistor parts is really what you have.
After putting thermistors of known value into the printer, I would then rebuild the Marlin firmware and upload it to the controller board.
It is very likely it will start reporting reliable results at that point.
After putting thermistors of known value into the printer, I would then rebuild the Marlin firmware and upload it to the controller board.
It is very likely it will start reporting reliable results at that point.