You will need reconfigure/change the Netgear router to be an AP using a static IP address; for example: 10.0.0.2 (if not already being used) per the first link provided in Post #2 above.
Reserve that 10.0.0.2 IP address on the Xfinity router using the Netgear router's MAC.
Connectivity (----> being an Ethernet cable):
Xfinity router [LAN port]----> Power Line Adapter ---to basement--->Power Line Adapter ----> [LAN port]Netgear router with DHCP disabled.
Netgear router/AP then serves wired network devices via its' remaining LAN ports as well as antennas for wireless network devices.
Correct and edit my diagram as necessary.
You do not need to use the router admin sites. Open a network browser and type in the IP address for the router you wish to access.
For the Xfinity router use 10.0.0.1 - you will still be prompted for login name and password.
Once set up, the Xfinity router manages all DHCP IP addresses to all connecting DHCP devices - wired and wireless. The Netgear is disabled and will not and should not provide IP address to other network devices.
Network devices will all need to be configured to obtain a DHCP IP address (10.0.0.x) from the Xfinity router (10.0.0.1, subnet 255.255.255.0) within the allowed DHCP IP address range.
That range could be 10.0.0.10 to 10.0.0.20 or higher depending on the number of of network devices expected.
Planning is important and I recommend that you sketch out a simple diagram of what you have in mind. The requirements.....
Overall once set up all devices will be on the same network independent of connectivity (wired or wireless). However, "roaming" may be and can be problematic thereafter.
However the first objective is to get everything up and running on one network.