I saw a clip on YouToob where two geezers built a cardboard and alumimum foil parabolic segment reflector which clipped over the antenna on the WiFi router and boosted the signal in one direction by some amazing amount - they claimed.
I have my router affixed to the interior wall at one side of the house with the antenna about 5cms parallel to the wall and all users are on one side of that antenna.
So I bought a sheet of aluminum, bent it into a very precise arc (no not really, I just guessed) and glued it to the wall behind the router antenna.
The before and after results in our bedroom*, the furthest room away from the router, are that I now have one additional bar showing on my smartphone screen. For about €3, an easy modification to try if your WiFi rig is at one side of the house.
This is what it looks like: the erect black object is the router antenna, the box what it emerges from is the router itself, and the curved bit is the aluminum sheet. Signals are reflected towards the viewer (as it were).
Is that explicit enough ? If not, photographs may follow (*not of our bedroom, though, it's too exciting).