@ClaudnDaye, that is not the case! The ??? error always means that the transmitter has a problem understanding (or getting) the sensor data. It never indicates a communication issue with the receiver: that kind of a problem shows up with a different error message on the receiver.
The step that Dexcom always recommends first is to change the sensor, not the transmitter. It is only after you had 2-3 sensors fail in the same way that Dexcom will swap the transmitter. We have had to swap sensors several times because of a ???, but never a transmitter (so far).
Dexcom always replaces the bad sensors by the way.