WiFi Extenders/Booster/Repeaters - What Do I Need?

you can use a wifi extender , it will need to be plugged in where it receives a good signal and then extends that signal
There are so many on the market ,
what country are you in?
Do you just want wifi in the "dead area" or do you also want to be able to use an ethernet cable

I use some BT wifi powerline extenders and have 2 extenders to cover the house and conservatory - they also use the electrical wiring "powerline extenders"

How big is this area in the apartment , where is the router located , remember the router will broadcast 360degrees , so you may be able to move the router to the other side of the room and get cover
what obstacles are in the way - things like water tanks and solid walls will reduce the signal a lot