Recommended for:
Players who don't see other players in the game.

How to do it:

Press Win + R
Type: cmd
Enter

Type: ipconfig
Enter

Type the default gateway into your browser address bar.
Enter

user: admin
password: admin
Enter

Now you are at the administration panel of your router.

Here come the tricky part.
Open this page: http://portforward.com/english/applications/port_forwarding/Call_of_Duty_-_Advanced_Warfare/

Search for the brand of your router and open it.
If you don't know the exact model of the router or the page doesn't list it just open pages until you see the same thing on the pictures and your router admin panel.

Navigate to the appropriate page in your router using the pictures of the other website.

Set protocol to: "all"

Ports: 25252-26252
or
Start port: 25252
End port: 26252

Local ip: the "IPv4" address displayed in the previously opened command line.

If the router asks for internal ports, then write the same ports as before.
If there is an "enabled" checkbox, tick it.

Press save or equivalent.