Sounds like one leg of the copper pair disconnected somewhere.
My broadband basic info/help site - www.robertos.me.uk. Domains, sites and mail hosting - Tsohost & Ionos.
Connections: OnePlus 8 Pro max 165Mbps down, 24Mbps up on Three, and B311 4G, tbb tests normally 35-45Mpbs down, 65Mbps off-peak, 9-24 up.
======================== Experience shows us that love does not consist in gazing at each other but in looking together in the same direction. Antoine de Saint-Exupéry.