Hi mikey
Many thanks
The browser at that moment is giving a correct error as your win10 pc is not on the same network as the hg612
So in a nutshell
Set the hg612 for 10.0.1.254 as you have done
Browser will error - shutdown browser
Change win10 IP address to 10.0.1.100 subnet 255.255.255.0 no gateway
Open a browser and goto
http://10.0.1.254Login
Confirm successful login
Plug hg612 into a LAN port on ATC
Change win10 IP address to dhcp
Check you can access hg612 from your Mac
Put your win10 pc into your not normal network
Check you can access hg612 from win10
If successful, plug phone line into hg612 and ATC WAN to LAN1 of the hg612. Leave hg612 LAN2 connected to ATC LAN port
Check internet
Everything should then be working
Many thanks
John