Not in this case Andrew:).
To get the stats from the modem you have them on the same subnet, bearing in mind the TP-Link will be in bridge mode. By connecting "modem" Lan 2 to any any Ethernet socket on the router you access it just like anything else on the Lan. It's the standard way we set up the HG612 with whatever router people are using.
Telnet and GUI access only.
Tim's powerline link will be purely to make that link. No serious traffic.
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 65618/13914Kbps @ 600m. BQMs - IPv4