Wifi not working more than 2 metres from router, signal strength good.

3 weeks ago I accidentally reset my router back to factory settings. Then I fixed it. After a couple days it stopped working, me and my flatmates couldn't connect. So I reset it again, this time knowing how to set everything. And now, I can only connect to the Wifi when I am less than 3 metres away from the router, which means not in my room. I have an android phone and a windows 7 PC and my flatmate has a Mac. The signal is good in all parts of the house, but it can only connect when the device is very close. What i've tried: Calling the internet provider - they told me to check the "internet density", and that from their side all is ok. I assumed he means the Wifi channels and tried changing them. This may have given me additional 1 metre but I'm not sure cause I was also experimenting with the antenna position. I upgraded the routers firmware. I tried changing channel width, encryption type, MAC filtering, antenna position, ssid, resetting to factory settings and powercycling (a lot of times). My network uses static IP. There is A LOT of other Wifi networks around, so my best guess is problems with interference. But I've checked several channels and none of them works well enough to reach my PC. Before it stopped working out of nowhere it could easily reach my PC and even sometimes the restaurant below my flat. The router is tplink. I dont know if that matters but about the same time it stopped working, some renovation started in the building. How could it have any effect? Tl;Dr. I can connect to my Wifi just fine standing next to the router but it drops when I move ~3 meters away. The signal strength remains excellent.