Tnx Steve, that was a helpfull hint.
I changed the router setup and connected the uRM with portforwarding rules from port 3837 to port 80.
Now I can reach my Arduino server as before in a browser without adding a port number.
From the outside world I can access the uRM with my_fixed_IP:3837 .

Right now when I connect the Arduino-server to the network and access it with a browser, the uRM ‘crashes’.
I can see on the local IP address 192.168.178.108 that the uRM went offline.
On https://www.uradmonitor.com/?open=12000059 the ‘Last instant readings’ does not update anymore.
5 to 10 minutes later the uRM reconnects to the network and I can connect again on 192.168.178.108 and a few minutes later the map will give a online sign again and the ‘Last instant readings’ updates again every minute.

As long as the Arduino server page is opened in a browser it lets the uRM ‘crash’ again and the sequence repeats itself as long as there is arduino-server-activity. In a browser the webserver site updates itself every 15 seconds with XML for time_information and some settings.

To me it looks like that the server traffic lets the uRM crash. But it may also be possible that my router goes nuts, i just do not know what and why this happens.

Coos