My TP link RE315 is making my wifi even straight to the main box worse.

I recently switched wifi provider from EE to BT which is the same anyway but since i got the new main box and set up my extender for the new wifi as you would and it seemed fine for a day or two then it randomly stopped working so i reset it and went through the whole setup again and since then if i connect to it, it is a bad connection and stops working randomly which never happened before, and now when i have the extender on but i connect to the main box instead it is having a bad connection when the extender is on, because when i take the extender out the wifi from the main box goes back to normal.

What do i do, i'm so fed up i'm thinking of just getting a new extender. Any help is appreciated, thanks.