Hello,
Using Private APN with Private IP as long as you have internet connection, should not restrict RMS access, unless yours Mobile Service Provider is blocking it. Can you access rms.teltonika.lt via browser when you are connected to internet with this TRB140? And can you ping it directly from router using this connection? If not, then then it is being blocked by yours MSP and you should contact them for RMS access with your devices using Private APN.
Best regards,
VidasKac.