I was finally able to host a co-op campaign game last night where my friends could see the server. My solution:

I added ports 27015-27016, 8766 to port forwarding in my router under UDP (previously I had them in TCP only).
I added a HDCP Reservation for my machine so it would always have the same IP assigned (I noticed it had changed since I last added port forwarding entries, so it was forwarding to the wrong IP!)

I assume my first attempt did not work because the changes to port forwarding caused a router reboot...which probably assigned me a new IP via DHCP when it came back up!

I hope this helps!