Hi Vlad from Russia here!! I'm with BT I had great wi fi for a year where i live then suddenly, it turned all crappy. I live in a block of flats and the lady upstairs switched to BT . it took me a couple of days to work out what was wrong, So i went through every channel writing good, ok, and...
https://www.youtube.com/watch?v=q0XXJesqgdQ
My advice would be to try every channel. right down the number of channel you find best then save it, piece of piss.