You can't set your router to a higher protocol than your ISP has set. You can only set it to the same as or lower. (You might make the setting in the router but it will always be forced to train down to at most the ISP's setting).
The implication is BT have you set to ADSL.
Connections: OnePlus 8 Pro, 4G+ (LTE) max 165Mbps down, 24Mbps up on Three Mobile, and B311 4G+ router, tbb tests normally 35-45Mpbs down, 65Mbps off-peak, 9-24 up (Three)ZTE MF286D router speedtest.net 113/20Mbps.
===========================================================================
The price of liberty, and even of common humanity, is eternal vigilance. (Aldous Huxley version of the well-known saying)
When you meet Mr Juncker, you realise you haven't got a drink problem. Nigel Farage, 12 Aug 2021