Generally (for Class II routers) it’s easier to use the ‘Rules’ on the Port Forwarding page rather than the ‘Custom Port Forwarding’ section (unless you need the extra configurability). First click on the ‘User’ option, then select ‘New’

Then enter the details like so:

Note:  The Port Start, Port End and Port Map will be whatever your situation requires. In this example, port 8080 is being pointed to 81.

Click on ‘Apply’ and then go back to the ‘Port Foward’ page by using the left-hand menu. Select the new rule that you have just created, click on ‘Add’ and then click on ‘Apply’

Assuming the LAN IP, port start, port end and port map are correct, that should do it!

At this point you may wish to go to the ‘System’ section and click on ‘Save All’ to retain the settings in the event of a reset.