Long term issue (months) internet drop out and wifi coverage

[Ponki]

This is a long term issue-over months the internet drops out, thinks/stalls every few minutes. Wifi coverage around our house with one booster doesn't reach upstairs or the second floor when the internet can stay in long enough. Sky technician has called  before....it used to be so much better. Additionally the Sky app broadband tester is comical, a tick for everything being just fine. It's not. Appreciate any help as I've got two students and a front line service worker who are really struggling. 
Thank you in advance