We moved 1.5 miles down the road & our broadband went from a consistent 6Mb to a regular 2.2-2.5Mb with the occasional 3Mb on a really good day.
From what I’ve read, it is due to the distance from the exchange & there’s not a lot we can do, apart from switch to fibre.
Or you’ve a fault with the house wiring. I used to get about that speed, then I fiddled with the house wiring and it went up to about 3-3.5 MB/s
We’ve tried the router plugged directly in as close to the main socket & then in the living room down an extension cable, which made no difference. We’ve also unplugged the extension, to see if that made a difference; it didn’t appear to. I’d be interested to know what you fiddled with to make an improvement, so I can try the same thing….