@neilh20. Hey Neil! Sorry it’s been a while, but just a quick update. I was able to get it to work. I ended up purchasing a new XBee cellular modem (same type) alongside the Digi IoT Development Kit. Works great now!
I’m thinking the issue was either that I had set up the modem with a board that wasn’t the dedicated XCTU board (I was using the Xbee Grove Development board). Either that, or I just messed up the set-up with the Xbee from the get-go.
Thanks for all your help!