The current Xfinity routers (end of 2017) do not support NAT Loopback and have no options for enabling NAT Loopback.
What is NAT Loopack?
NAT Loopback or Hairpinning is when Device_A behind a router tries to access another Device_B behind the same router by Device_B’s external name (e.g. domain name) instead of its local name (e.g. local IP address). Because of the way packets will leave the router and reenter, some routers disable NAT Loopback for security reasons.
Problem Description
When a router does not support NAT Loopback, a techie will have a problem if they are:
- hosting a home server
- using a domain name to route to the home server
- accessing their home server via the domain name while on the same network as the home server
Partial Solutions
- While home, access the server using the local IP address.
- Use a VPN to route traffic out of your home, which will then go back home and to your server.
- Buy a router that supports NAT Loopback. [link_1]
- Set up a local DNS server to change the public domain name to the local IP address.
References
- https://forums.xfinity.com/t5/Your-Home-Network/From-within-my-home-network-external-router-IP-does-not-work/td-p/1474663
- http://www.dslreports.com/forum/r26574730-Connectivity-NAT-Loopback
- https://www.reddit.com/r/servers/comments/4r3or6/nat_reflectionloopback_on_a_comcast_modem/
- [link_1] http://opensimulator.org/wiki/NAT_Loopback_Routers
- https://en.wikipedia.org/wiki/Hairpinning
Man and Keyboard 
Download Opera, turn on VPN in settings, then when you paste your Wan IP it works.
(Using Windows 10 with Windows Web Server Services)
This fixes the Nat Loopback(HairPinning) Problem. Other wise do a seach for Port fowarding.
Anytime of VPN works in the browser or on the desktop. But without a VPN, NAT Loopback is still a problem.
Two solutions to the lack of NAT loopback.
1) Use the hosts file and add the computer name and IP address. On a Windows computer, the hosts file is located in C:\Windows\System32\drivers\etc\hosts. Windows 10 users will need administrator access when when saving the file. Right-click on the Notepad app and select ‘Run as administrator’. Add a line similar to this example to the end of the file.
192.168.0.150 http://www.myserver.com
192.168.0.150 myserver.com
Replace the IP address and computer name with the ones for your computer. Note that you need two entries so that both forms of the server name work. (with or without www)
The hosts file is used before DNS lookup, so this will point your browser to the computer located within your local area network without the need for loopback. You will need to do this on all computers within your local area network (LAN).
2) Your router may support DNS host mapping (it may use a different name). Host mapping works similar to the way the hosts file works, but the setting at your router is global for your LAN, so once it is set, all computers on your LAN will be directed to this connection.
Correction to above post. Do not add “http://” to the line. This was added auto-magically because I used ‘www’.