Just got the UBR specifically because of support for the 600MHz, which is the only usable T-Mobile band in the area where I am eventually using the product. I also had an ATT SIM and a Verizon SIM to test with. All SIMs worked fine in cell modem A. On cell modem B (the only modem with 600 band) T-Mobile would attach to the LTE network, but would never obtain an IP address, even if I forced it to all the different T-Mobile bands/3G. I called T-Mobile to troubleshoot, and while I was on the phone, on a whim, I switched the network mode to AT&T. I had already tried T-Mobile mode. It came right up and worked fine, shows it’s on the T-Mobile network. I even tried using Verizon network mode, and it also came up. Hopefully this saves someone else some time.
Thanks! This solved the problem for me also on a balance 20x. Seems like a bug they’d want to fix.
Firmware version 8.1.0 seems to fix this problem. This version may have been released today.