RB2011 on ATT 4G Not Showing LTE Interface

I recently fired up an RB2011 using a Sierra MC770 and a known working USB adapter. On ATT’s network, instead of getting a 4G LTE connection, the interface shows up as “ppp-out-1”, despite the fact that I have 4G coverage when using the same device via the PC.

Any ideas why this would be occurring / how to fix it?