Go the webpage for the router & login to access the settings
Navigate to 'WAN' on the left sidebar, then into 'General Setup'
Select 'WAN 2' & change the 'Enable' dropdown to Yes
You may need to change the 'Physical Type' setting based on the device the router will be connecting to
Click on 'OK', the router will now need to restart to apply the changes, so click 'OK' once more
Once the router has finished restarting, navigate back to the 'WAN' section in the left sidebar and then to 'Internet Access'
For 'WAN 2' change the Access Mode using the dropdown to fit your required configuration
Then navigate to the 'Details Page' button to the right of the dropdown
You can then adjust the settings to fit your configuration, for example enabling DHCP and settings the IP Address to automatically obtain.
After you have completed all necessary configuration, click 'OK' at the bottom to save it. You will have to restart the router again byt clicking 'OK' once more