[werk_ag]

... to be able to look at it directly from any other IP and use the GUI from Cumulus?

Allowing outside access to the Cumulus Web GUI is not a good idea, as everyone will have access to the configuration pages. There is a lot od discussions about this matter on the Cumulus forum.

The best way to do what you want, is use the RPI as a webserver, and serve the default Cumulus webpages from it, or even use the Saratoga Templates. If you don't expect too much simultaneous accesses it works pretty well.