I've setup a port forward, and am able to use this successfuly from a computer not on my local network. However, when I logon to my network through WIFI or wired ethernet connection to the router, the forward does not work. Any ideas?
Not sure but Google for NAT loopback / hairpinning and see if it applies to your situation.
So do Xfinity routers have this issue of blocking local traffic directed through the remote address? Can it be turned off? How?
Yes. They do not have a toggleable NAT loopback feature. It's always disabled.
No. It's hardcoded into their firmware due to its inherent security risk.