WiFi boosters

Hi Susan15, buy "powerline adapters", best of the brand "Devolo". You can get them at Worten, RADIO POPULAR; FNAC and similar shops or online. For installation follow the installation manual. The power line adapters use the electric house grid to transport the signal/Internet whereever you need it. You always need minimum a pair (a so-called kit). 1. A sender: it is connected to the router via Ethernet cable and plugged into the next wall socket. And 2. one or more  receivers/outlets that you install in every room where you need proper Internet. Choose the WIFI ones. They come also with a Ethernet socket, so that you can connect your laptop or computer also via Ethernet cable when- and wherever you want in your house. If you feel a bit insecure better buy it in one of those shops and ask the people there.
I have this kit here installed in my hous: https://www.worten.pt/produtos/powerline-devolo-magic-2-wi-fi-next-starter-kit-7167707

The kit comes with a sender and 1 receiver/outlet. You can add more outlets if needed (like in my case).

But don't buy the brand Linksys and also not TP-Link, because of their relatively poor quality. And don't buy "repeaters". They look very similar, but they don't help a lot. 

I hope this helps, regards, Ecki