Hi Rick
Sounds like you’re doing everything right.
After reverting a mDot to an older revision an AT command is not required to complete the software revision.
mDot serial number 19056321 shipped with firmware 2.0.16
mDot serial number 20121591 shipped with firmware 3.1.0
It is unclear why you can not communicate with the mDot in the developer board using AT Commands.
Are you able to successfully issue AT Commands to 19056321 in a developer board?