Bad internet signal

The signal is too weak. You are probably correct about the wall being the cause, especially if it is a material that is a serious obstacle to Wi-Fi. If the path from router to computer is diagonally through the wall try to move one or both to make the path be as perpendicular (hence shorter through the wall) to the wall as possible.

If the distance from router to computer is also a significant factor (not likely) then the Extender may help. You would put the extender either on the router or the computer side of the wall. To test before buying temporarily place the router or computer at the location where you would put the Extender. If you get a signal at least -70 dBm and can connect OK then the Extender will probably help. If the signal is still too weak then you can be nearly positive that the Extender will not help.

Maybe there is a way to run an ethernet cable through or around that wall? Along heating/cooling duct or electrical wiring? By detouring to the basement or attic?