As was already stated in the problem description, I would again check the positioning of the router. Are there any objects, big or small, in the way of the signal going to the second upper floor? Such objects could interfere with the signal strength. Think of the Wi-Fi signal as beams of light originating from your router that get reflected on each surface until they finally reach your PC or laptop and try to optimize it that way.
Furthermore, you can check your router’s settings for it’s channel settings. To prevent interference from your neighbours Wi-Fi sources the channel settings should be set to “Auto”. You can also try to set it manually, but I would recommend scanning all for Wi-Fis beforehand, with an app like Wifi-Analyzer.
There are also hardware options if you are willing to further invest into better Wi-Fi coverage. Simple Wi-Fi extenders like this one can be placed around your flat to amplify your signal by placing them at key points, e.g. near the stairs where signal might still be acceptable. Also, there are Power-line adapters that modulate your internet communication onto your power lines within your home. These can be hit and miss and depend on your home’s power lines.
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>