The following guide is intended for the latest official firmware (TD-8840Tv4_0.8.0_2.3_up(150917)_2015-09-18_09.14.43.bin) which you can download from here.
1 Log into your router with your username and password (default IP: 192.168.1.1 - default username/password: admin)
2 Click on "Forwarding" and "Virtual Servers" in the main menu on the left
3 Click on the "Add New" button
4 Select "HTTP" as "Common Service port" and enter the (preferably static) IP of the computer cFos Personal Net is running on in the "IP Address" field
5 Click on the "Save" button
6 Log out of your router by clicking on "Logout" in the main menu on the left
Port forwarding is now configured for your computer!