Hi Pauliusand and Daumantas,
my current firmware version on my X11 is RUTX_R_00.07.04.4.
Most probably I did the update from the last stable version as I regularly do the updates (but I cannot proof the version no. before).
Please note that I today examined that there also was a firmware update pending for the internal modem (shown in your “Update Firmware” section within the Teltonika WebUI). Therefore I successfully updated this as well (via server option) to the latest available version EG06ELAR04A20M4G. Kernel version is 5.4.229.
Also I proceeded the commands for MTU change as per your proposal and got no error messages. Therefore these changes seems to be operational now.
I will give a response a.s.a. the problem occurs again and also in case everything would run fine after let´s say 24-48h.