is it possible to post connection stats from your modem/router so we can see and help comments
did you use the link posted by @iniltous as that will show what is available to you - can you post results
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
Thanks all - I am not at the property but I will certainly post some stats including the original router stats as soon as I can.
Thanks for the help everyone