Had (have) similar problem. Was using only 4G in Bulgaria with A1 (former Mtel operator) and at one moment Could not anymore work via 4G - just 3G and 2G. Sim Card from the router in my phone is working fine on 4G.
Factory reset did not work for me
For me the bootloader was 2.00 and
Once I did upgrade first the Bootloader to 3.01 - it connected 4G in no time. Of course ..I moved to the latest Factory Firmware, but was impossible to me to make it work .Probably some setting I could not manage to put properly. Tried randomly with some of the firmware versions 4.xx and 3.xx - but did not connect to the 4G - Status "disconnected"
The 2.xx FW are not available on the Teltonika Website :( it was hard to me to find the 2.xx FW to test and finally when I put this ver.RUT9XX_R_00.02.818 it worked immediately again. No idea what happened after that maybe going further with the Router configuration.. but it keep showing for the 4G status Disconnected.
any ideas are welcome