Since few weeks, my brother getting some trouble in his chamber with wifi, he cannot connect to the wifi because the walls of the house is terribly thick and heavy.
So are there any devices which improve the range of Wi-Fi. Should I get an AP, a new routeur, or something else ? I'm looking for a product with the best performance/price.
2) Consider turning off 5 GHz completely. 2.4 GHz has better penetration through walls and other obstacles, and the speed difference between 2.4 GHz and 5 GHz when you're closer to the access point is often irrelevant compared with the speed of your Internet connection.
3) If you buy additional equipment, do not buy a "repeater". They're usually crap and usually make things worse overall. Buy a device that can be a bridge - that is, that can act as an access point and not just as a "router" - and connect it to your primary device via ethernet, and situate that device closer to where you're having issues.
This does not directly answer your question and it may not be an option, but if you can run an ethernet cable directly from your router to your brother then he could plug in directly or use a really cheap wifi at his end if he is not more than 100 meters away. A drill and caulking agent can get through the wall if it's not a rental or sneakily out a window/under doors/through vents if it isn't obvious to neighbors or landlords and not a tripping hazard.
Or try gigabit powerline