Sounds like the computer isn't behind a NAT, so port-forwarding is unnecessary.

While quite unusual for home connections, it's technically perfectly fine to not have an "internal" IP address – home connections only use NAT and port forwarding in order to deal with IP address shortage, not because the protocol says so.

The computer might still be behind a firewall, however, and you might still have to add an "allow" rule.

  But first you'll need to investigate how everything is connected. Is the computer connected through that router, or perhaps it's the other way around?  (Some people just enable "Internet connection sharing" and use their Windows PC as the main router... And the device you call a router might be currently acting only as a cable modem, or as a WiFi access point, not necessarily as a router.)

Sounds like the computer isn't behind a NAT, so port-forwarding is unnecessary. The address you see in ipconfig is really the computer's own address.

While quite unusual for home connections, it's technically perfectly fine to not have an "internal" IP address (and it's how things are always done in IPv6) – home connections only use NAT and port forwarding in order to deal with IP address shortage, not because the protocol says so.

The computer might still be behind a firewall, however, and you might still have to add an "allow" rule. But first you'll need to investigate how everything is connected in order to know where to do that... 

Is the computer connected through that router, or perhaps it's the other way around? 

(Some people just enable "Internet connection sharing" and use their Windows PC as the main router... And the device you call a router might be currently acting only as a cable modem, or as a WiFi access point, not necessarily as a router.)

Is that address actually provided by AT&T, or is it from a VPN company?