Wi-fi coverage
Here are some solutions that I have used, that you might like to consider.
A) Move the Router to the middle of the house, E.G under the stairs. Sometimes this is enough to give coverage to the whole house. You can test the coverage by just moving it and plugging it into power, and seeing what the range of the signal is (even if it's not connected to the internet)
If the range is good enough, you’ll need to have a way to run the cable permanently , i.e. under the floor etc.
B) Power line adapters, having hated these for years… I’ve used a couple of Netgear ones (PL1000 and PLW1000) recently and was surprised how well they worked. You can usually find a pair on ebay <£40 so worth a try. (You plug the PL1000 into the mains and the router and the PLW1000 in the “destination” room. (There's probably a newer model now)
C) Depending on the Router supplied by your ISP, you may be able to buy, or pay an additional fee, for "extenders" that connect to the hub wirelessly, also known as a mesh network
D). Depending on the ease of access running ethernet cables from the router into the “offending” rooms and adding an access points works best of all, I have done this many times. Usually under the floor, or by running the cable along an outside wall.
E) You can also replace the router with "something better", which may have more range, better performance and features.
After many years of struggling to get coverage and reliability, I discovered Ubiquity uni-fi, which has been fantastically good, it but required significant more money, time and knowledge to set up. It's s more commonly found in commercial premises. (which is probably why it works)
Good luck.