There are many reasons why a speed test won't give you what you think you should be getting. The first thing is that you should ideally be doing the test on a device connected by ethernet cable to the router - wifi will almost always be slower. You should also make sure that other devices are disconnected as they will absorb bandwidth - this includes smart TVs and dongles (Chromecast, Firestick etc).
The only true reliable test is line speed, which you can't do. If the speed supplied is as advised, then it's correct and I'm afraid anything less will be due to your environment unless there's an actual fault with the router, which presumably has been checked and elimininated. Speed tests on wifi should always be conducted close to the router and allowance made for the maximum speed the device and wifi protocol can accomodate.
I also assume that a line fault has been ruled out? I've just had an issue with that. It was intermittent and the speed tested OK when an engineer was present, but could be noisy at other times, causing drop-outs. I persisted and the fault was eventually located at one of the external junctions and fixed. You can test for noise by plugging a phone into the master socket and listening for a clear dial tone.
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