After Reboot the connection was lost. As soon as we changed something in the configuration of the RS232 (like echo on/off) and saved it it worked fine again until the next power off.
Current FW Version is 00.05.02.6. We tried upgrading it to the latest FW Version (00.05.06 I think) but the mobile communication was not stable afterwards. So we went back to the current FW Version used.
For the Mobile Config we only set up the APN as we use industrial SIM Cards (No PIN) for this test.
This is the current configuration of the RS232 interface. We simply want to send the Serial Data Over IP to our Server via TCP Port 80. In our application it does not matter if we allow any IP Addresses over other interfaces like LAN so we left that part empty (this was tested too).
Lastly an overview of the router. Thanks for the reply.