My ISP is AT&T

AT&T routers often do not have UPnP feature and port forwarding has to be done manually. 

For example on BGW 320-505 / 500 AT&T Gateway / Router, you need to manually set the port range for port forwarding based on the images below:

Step 1: Add port ranges to be forwarded

Step 2: Bind service to device

This video shows the steps with more details.