Before we start, let's look at the configuration we're trying to achieve and the prerequisites that make it possible. Prerequisites:
A RUTxxx series router with more than 2 ports.
At least two cable internet connections.
A final device to test the configuration.
Configuration diagram:
Router configuration (WebUI)
Connect to the router's WebUI, go to Network -> VLAN -> LAN Network, and create a new interface by entering the name and clicking Add New.
After clicking Add New configuration popup window, a default window will appear, press Save.
After saving the configuration, you will be redirected back to Network -> VLAN -> VLAN Networks, now you should open the VLAN Network tab in the same window and you should:
Select VLAN mode: Port-based
Current LAN interface:
Enable wireless access
Select your current LAN interface.
Disable LAN port 3 (which will be used as a WAN port)
Press Save.
Note: Make sure you are not connected to that LAN port you are going to disable.
New LAN_WAN interface:
Click Add and a new row will appear.
Turn off LAN ports 1-2 and leave only LAN port 3 enabled.
Select your newly created interface in the LAN section and click Save.
Test the configuration
If you have performed the steps correctly, your final device should have internet connection according to the diagram.
Go to Network -> WAN and verify if the new LAN_WAN is enabled and if it has an assigned IP.
Configure your WAN as the primary connection.
Configure your LAN port configured as WAN failover backup.
Go to www.whatsmyip.com and verify your public IP.
After that, disconnect the WAN cable and check your public IP again. Your final device should have internet connection and a different public IP.
So, reconnect the WAN internet connection and wait a few seconds. The public IP should change again to the first shown IP.