Open your browser and go to 192.168.20.1
Note: There are currently two firmware versions for this router, if yours is the latest one (blue background) just set as follows:
Otherwise if it has the older menu then proceed with the steps below.
Step 1: Advanced Setup > WAN Service
Step 2: Remove all Entries by ticking the remove checkbox, then the removal confirmation button below
Step 3: Visit Advanced Setup > Layer2 Interface > ETH Interface and ensure you have this value, otherwise there's an add button to add the ETH4 interface
Step 4: Advanced Setup > WAN Service click add and select ETH4 before pressing Next.
Step 5: Click IP over Ethernet and click next.
Step 6: Select Enable IPv4 for this service and Optain an IP address Automatically:
Step 7: Select Enable NAT and Enable Firewall:
Step 8: Ensure ETH4.1 is listed on the left under 'Selected Default Gateway Interfaces', then the same again for the DNS page after.