Updated my RUT240 to the 00.07.01.2 in two steps as informed (from initial FW to 01.14.5, and then to 00.07.012)
After the performed updates, it seems the AUTO APN for AT&T is missing an APN and therefore my mobile broadband SIM-card would not connect. It had been working perfectly before. Luckily I found an instruction that wrote that you have to add "broadband" as an APN, and then it worked. Strangely enough I do not know why Teltonika erased that APN as one of the AUTO APN options for AT&T?
I usually always work remotely against the Teltonika telling friends about it, but this problem forced me to make a 4 hour drive to fix this issue, not happy about it.