And having put the R850 back in service you have connected the Hive Hub to it?
Yes, and it didn't work... although... not at the time.
So having done what I described above I went out for the afternoon... a few hours later and the Hive hub seems to be working as normal... i.e. i have access locally?
So the things I would normally do now would be to change the HG612 modem IP address from the default 192.168.1.254 as it clashes with a work VPN and also I would fix other IPs for my NAS and Home Alarm panel.
Also I have not not connected the ethernet cable back up to the LAN2 port on the HG612.
I assuming one of those things is the issue? I definitely need to change the modem IP and I dont see why fixing other IPs should cause a Hive clash?