I have had the same issues since Virgin updated the firmware of my superhub 3 about two months ago. My iPhone was connected but it on a speedtest was getting almost nothing and it would stop half way through the test. The old iPad with old firmware was working perfectly.
I have a Fingbox that plugs in to my router and monitors my connections etc, anyway according to that my iPhone 11pro was on channel 6(2.4Ghz) but when I logged into the router it said that my iPhone was on the 5Ghz network, as I don't really need the 2.4Ghz network I have disabled it and it has solved my problem.
I would assume giving the 2g and 5g networks a different name would also work.