Learn from my blunder and don't trust a speed test run over wifi – use an ethernet cable to be sure. In years of ADSL I read this advice countless times but found it made no difference on my ADSL connection which was rock steady at 12/13Mb depending on congestion.
Then I upgraded to fibre and was disappointed when I got only 20Mb. Before complaining to Plusnet, and dimly remembering the advice (even repeated in my new router book!) I dug out an ethernet cable from an old router and presto – 37Mb !! Clearly the ISPs know what they're about when they advise you to test with a cable before pressing the complaint button.