Thanks for the input. With "NTP server enabled" and the adjustment of the NTP server on the M-Bus-Gateway (IP RUTX08) the time is now synchronized.
I hoped that with the solution of the "time sync problem", the actual problem would be solved as well: The M-Bus gateway sends emails via a predefined smtp server via port 2525 (Error connecting to server: ... on port 2525). Again: without RUTX08 it works. Do I have to enable something on RUTX08 as well?
Interestingly, I have already made different identical installations (RUTX08 and M-Bus gateway but different providers) and there everything was "plug and play".