The suggested solution from Teltonika has worked: A hard reset to the default settings and a new configuration of the rules - seems that an update to a newer firmware mixed up the settings on a deeper level, invisible via the interface ...
However you will lose all settings (eg. altered IP adresses etc.) - if you have only remote access to the router, you will probably lose the access itself. So it seems a good idea to activate the RMS access via Teltonika before the reset: You have to create an account at Teltonika, then you activate the RMS management in the router settings, and finally you can add the router to the RMS, which is free of charge for one month - after this the access will be deactivated via RMS, if you don't want to pay for further access. But the great advantage of this solution is the possibility to maintain direct access to your router separate from all resetted settings, so that you can reconfigure it to your needs ...
