TBH , you are not supplying the info requested so it’s impossible to give anything further that would be worthwhile, you obviously could report a ‘fault’ , but it’s not obvious that you have a fault , remote testing could show something obvious, or may simply say everything is testing OK , and with you not using a BT supported router the chances are any further remote diagnosis would be limited .
At a minimum, you need to know what is the speed range predicted for your phone number or address is , if there is a recent observed speed recorded , is in line with that , if the observed speed is significantly under prediction, then investigation is warranted, but the issue could be something you are responsible for…at the moment no one can offer any insight as the required info hasn’t been given, personally I think you should remove your third party router and even if only temporarily reconnect the BT router , then post the router statistics
