@Paul_M I assume you are using an LTE Boron in which case when downgrading from DeviceOS 2.x+, you’ll need to go through DeviceOS 1.5.2, otherwise DeviceOS will be unable to communicate with the cellular modem. After downgrading to 1.5.2 (and attempting to connect to the cellular network), earlier releases can also be used.
We’ve updated the release notes on GitHub and Particle Device OS Updates Thread with this breaking change.