The solution was hard to find and quite embarissing.
It was so strange, that I only could connect to the access point, but not to any other clients and not to the A1 modem. I checked all cable connections (it was a dark room). I found out that a black thin ethernet cable was connected in port 1 and 2, creating a loop.
I asked my sister about it and she thought that there is a loose cable that should be connected, but was there for maintainance purposes.
Due to the fact that the interface seems broken you have to use telnet. Tough it establishes the same mappings via telnet they do! work.
Steps.:
Using telnet
=>service host add name myService
=>service host rule add name=myService protocol=udp portrange 27000-27030
=>service host rule add name=myService protocol=udp portrange=28960
=>service host rule add name=myService protocol=tcp portrange 27014-27050
=>service host assign name myService host <192.168.0.2>