Need advice on how to better internet coverage

Hello,

I’m on a 500mb package but the wifi just doesn’t connect in quite a few parts of the house. I enquired about adding total wifi to my package but support said that new package would total to £31 a month whilst i’m currently on £17 a month so that isn’t an option for me

I’m looking at a mesh system like TP Link Decos whilst using the existing router as an access point. But I also have a home phone line as part of my current package and am wondering if this would still work on the existing router? Unfortunately the home phone line is a requirement

Would appreciate any help, thanks