I recently moved home and got FTTC with PlusNet, the speed is good when measuring (almost the advertised 80Mb/20Mb) but the connection was unusable due to TCP connections hanging every few minutes (very annoying with ssh but screen helps, worse when using a website for a payment and needing to retry and trust you will only be charged once).
Yesterday I decided to sit down and investigate. Router has logs which were quite helpful. A lot of things like OUT: BLOCK  Packet invalid in connection (Invalid tcp flags for current tcp state: TCP [192.168.1.73]:54426->[188.8.131.52]:443 on ppp3)
This followed the laptop being seen moving from interface ath10 to interface ath00 and it was moving back and forth quite often.
Looking at the logs on one of the laptops those switches looked like wlan0: disconnect from AP b8:d9:4d:41:76:fb for new auth to b8:d9:4d:41:76:fa
What happened is that default settings on PlusNet router is to have “identical” 2.4GHz and 5GHz networks so the devices believe they are the same network and switch between AP, but they are actually different and the connection tracking gets reset each time such switch happens.
Disabling the 5GHz network made my connection usable, I could probably just change its settings to make it separate.