since joining BT approx 6 months ago, I have nearly always had the correct speeds off 400-500mbps. In the last 2/3 days this speeds are only reaching 90mbps.
I have the router connected to the ONT through and Ethernet port in the wall behind the TV. I have tried connecting the router directly to the ONT and the broadband speeds go back to normal (500mbps).
why all of a sudden has the port behind the TV now slowed down my speeds?
Assuming you have some sort of patch panel , and normally connect the ONT via RJ45 to the patch panel and the Ethernet socket behind the TV connects to the router via RJ45 , it could be one of the RJ 45 cables ( depending on which one you used when testing with the ONT directly connected to the router it can’t be that one ) , or it could be one of the Ethernet sockets , the one at the patch panel or the one behind the TV , or even the cable that connects the two together.
If this is a relatively new build property, often the developer provides this ‘network’ at first fix , so the cables are hidden behind the internal walls , and if that’s where the fault is , fixing it could be impractical , although it’s not common, things can simply fail without warning.
You've tested direct connection to the ONT, now try the same direct to the router using the same port as used for the patch cable to the port behind the TV, then test the patch cable in situ.