The house of my parent’s in law seems to have WiFi gremlins living inside.
Even obtaining a decent signal 8 meters from the router in the past proved to be a challenge.

As a first attempt to connect an old desktop of mine I tried a TP Link WiFi N PCI adapter (TL-WN751ND) with a single antenna (the best that was available in the nearby computer shop): the router signal was detected but connection always failed.

To improve the situation I’ve added a high(er) gain antenna (8db) with an extension cord: signal appeared significantly stronger but connection was still failing most of the time and when successful was lasting only a few minutes.

As a last attempt I decided to convert the 3G router that I already had from the stock firmware to the OpenWRT firmware and use it as a bridge providing ethernet connectivity to the desktop.
Installation was very simple as it worked from the standard firmware update interface of the stock firmware and in about 10 minutes I’ve had the bridge up and running.

Most likely the big challenge is related to signal scattering and the 2×2 MIMO did the needed magic.
Now the connection is fairly stable and I can get the full speed of the ADSL line to the desktop.

A big thank you to the OpenWRT guys.