Most ISP supplied routers have poor WiFi with an omnidirectional internal antenna and then people go stick them where the computer is, tucked away in a corner of the house, or by the master phone socket, often likewise at a corner where the cable is easiest to enter the house. Worst position for WiFi to spread around the house.
You can get an access point and either turn off the WiFi in the router or keep it on and have two APs. You can use the same SSID & password or different. Same means not having to configure each device for the new AP.
Locate in a central point in the house, higher up and ideally one with external antenna. There are various tricks to make it more directional rather than radiate half the signal out of the house.
Been looking at a solution for my parents, and could either wire up with cable or use a powerline thing with a WiFi AP at one end. Can also get WiFi repeaters although they’re less advised as they halve the bandwidth apparently.
Other simple option is to relocate the router and see if that helps.