I have an LTE Cat 1 Verizon, and I'm using an Arduino Due as a USB-to-serial converter for communication. That setup worked fairly well until the XBee stopped giving correct responses to commands. Now, if I send anything over serial (including +++, any AT commands, and random characters), it responds with a three-byte sequence: 0x1C 0x1C 0xFC, or sometimes that sequence twice. I see no correlation between the input and whether the sequence is sent once or twice.
I'm not sure what would have caused this. I changed the BAUD rate to 115200, but otherwise I don't believe I made any configuration changes recently.
I tried entering command mode by sending a serial break for 6 seconds, but that appeared to have no effect (the BAUD rate is still 115200 and I never got an "OK" indicating the device moved to command mode).
I also tried a factory reset with ATRE followed by ATWR, but this appeared to be ineffective as I don't get a valid response to any commands.