Just to clarify, leaving the modem alone, simply swapping the router causes the speed to increase again.
This means there is nothing wrong with the sync. The FTTC is not disconnecting. It is a loss of the PPP session it seems if the "internet" light on the Plusnet router turns red.
Have you tried just turning the router off for the time it takes to swap them, but then just turning it back on without a swap? As opposed to the reboots of it you say you have done.
If router power off for a few minutes restores the speed this helps diagnose where the problem could be. I'm beginning to suspect where your fibre circuit is handed over to BT Wholesale at the exchange.
Kindness isn't going to cure the world of all its awfulness but it's a good place to begin. Daisy Ridley.
My broadband basic info/help site -
www.robertos.me.uk. Domains, site and mail hosting -
Tsohost.
Connection - AAISP Home::1 80/20. Sync 59546/15321kbps @ 600m. -
BQM
Edited by RobertoS (Wed 11-May-16 23:37:12)