Hi Martynas,
I unregistered the RUT from the RMS and registered again wth the s/n and MAC that appeared on the box (the MAC was 1 digit different from the previous one).
Now the error message says: Failure (Error: Failed to resolve hostname.) The hostname that is shown in RMS Settings is: rms.teltonika.lt
The serial number of the router is: 1102120119.
Thanks for your support