Back to full speed
It took two hours but I did get a new modem/router up and running. Internet speeds have returned to a steady state which proves beyond a doubt the old one was failing.
My overall experience with modems hasn't been a good one. Notwithstanding the change from ADSL to the NBN, that's now 6 modems in less than 7 years we've lived here. Other houses were the same. They are one piece of tech that seems to degrade over time.
Still have some issues with not begin able to see all services within the house. Can't tell if its a different firewall trigger, or there was a setting on the old modem that I have to replicate.