If you are transmitting the signal through the wall in a steep angle that wall becomes towards the router a very thick wall.
Any metallic structures can influence the signal as well. As there would be framing within wall or ceiling. Steel beams or steel concrete structures. Or aluminium (tin) foil (for example at the back of the plasterboard).
Cables and pipes not to forget.
If the place is multi storey then the aerials tend not to reach up or down very well.
Then you may get problems with EMR (electro magnetic radiation) which had been mentioned by the cordless phone already.
But again there are many more sources like cables, High voltage lines nearby the premises. Earthing been carried out insufficient so that the HF of your switched power supplies spreads all over the place. A radio station nearby. Or any other heavy industries.
Not to forget if there are more wireless networks in the neighbourhood. Or baby phones. ...
But I should have asked first, are you loosing the signal completely or is the data rate dropping badly after that?
Now the router is only one part of the story. Your wireless device is the other participant.
How did you rule out the problem on that end?
The router is only a wireless G device with a single aerial and no connection for external aerials.
Which makes it more vulnerable to environmental influences than modern wireless devices.
Wireless networking is a very complicated area. If it works, good. But if not and you have to rely on it then that can cost you thousands to find the reason and depending on the solution quickly hundreds of thousands to alter the building to suit the wireless network.
But as a quick measurement just move the router to a different place for testing purposes (away from building structures).
And see what influence that makes.
If the problem still consists then rather have a thought if cable network could suit you or if you need additional Access Points. 5m is not bad after all in a problematic building.
