ditto what TheEthyr said Re: your ISP configuration simplifying the MoCA setup … but even greater simplification and optimization is possible if just trying to link one remote room to the router LAN; see the following parallel reply by TomRILReddit from today for another thread attempting the same…
I would like to repurpose a Netgear AC1200 Modem-Router as a range extender/
MOCA/Switch to add ethernet ports to our home offices.
Easy enough to test: Can you configure the Netgear modem/router in such a way that allows it to be hard-wired via Ethernet to your Xfinity gateway, LAN to LAN, such that any devices connected to the Netgear, wired or wireless, receive IP addressing from the Xfinity gateway? (typical manual configuration involves setting a manual IP for the AP, plus disabling DHCP services)
If the Netgear modem/router can’t be configured to work in this way, it won’t work if linked via MoCA, either. (Best case, you’ll need a separate MoCA adapter to proxy the Netgear connection with the Xfinity gateway.)
Or a damaged Ethernet port. (Agreed that the drop to 100 Mbps max seems too coincidental to be a MoCA/coax issue … at least for the purpose of prioritizing troubleshooting.)
Does the router offer a dialog where the link rates for the Ethernet WAN port and Ethernet LAN ports are reported?
Who’s the provider? Comcast typically will perform this service free-of-charge; that is… grouping and isolating the residence’s coax behind a “PoE” MoCA filter.