This doesn‘t work this way. You have to create a seperate Interface (call it WAN2 e.g.) in LuCi → Network → Interfaces, you most likely need to set it up as DHCP Client (depending on your WAN connection). Enable „Use default gateway“ in „Advanced Settings“. You can assign it to firewall zone WAN for simplicity, then you remove LAN4 from LAN bridge and ad it to the newly created Interface WAN2. You can control which WAN interface should be default by assigning a lower metric in its „Adcanced Settings“.
With this configuration you should have internet access with both or either one of the „WAN ethernet cables“ connected. If you remove one the corrosponding interface goes down and traffic goes through the other. Problem is that not all internet outtages lead to the interface going down. This is why you need to set up something like mwan3 that regularly checks both wan connections and changes routing table if an outage is detected.
This is impossible because eth0 is directly connected to omnias internal switch. You can‘t change that through configuration without soldering ;).