Hey Mike --
I don't know... First time I put a sensor on, I was sort of timid about it (reminding me of my first injections back in 1982...). When you put the sensor on, you just gotta do it, emphatically (not rushing it... just DOING it, y know?). I don't know if that was your situation, but if your start was anything like mine, that could be the case.
So, you might want to call Dex and tell them of the constant '???' and they may tell you to try another sensor, at which point you should ask if you can get this wonky one replaced, no matter the cause of the wonkiness. More than likely they will agree to replace it "this one time" (as they did for me right when I started). I have seen that others call in several times and get them replaced with no issues, but they told me it would be just the one time.
Usually takes a couple of hours before my "???" goes away and I start getting real numbers again. It doesn't clear right away.
That stupid alarm.
In a clean environment - no cell phones and no loose RF; I usually put receiver in fridge or wrap with alumininum foil till freceiver shows disconnected.
I then let it loose again and if sensor is stable ( and usually is) receiver should recover in 30 minutes. If still trouble, get in your car and go somewhere else for another 30 minutes.
Repeatedly, I found myself recovering unit reliably in 30 minutes but key is to force disconnect of receiver - antenna status by covering with foil and/or stuffing in fridge.
There seems to be some very stupid code and latch up status. Also shut down cell phone if its with you.
Did you try using the "stop sensor" command, and then the Start sensor" command. That is like rebooting your computer to get a fresh start. I have not had a ??? that never went away, but that is what I would do if that happened. I hope you will find something that works. It is important to check the transmitter and be sure that both corners of the wider end are fully snapped into place.