Sorry, my mistake...model is TL-R600VPN.  Here is a pic of the connections.

@georgiejr 

  I have the TL-R600VPN and tried using the Link Backup feature for WAN backup. What I did not realize is the the Backup WAN is automatically shutdown if the Primary(s) is available when Link Backup is configured. I need to use both WANs if both links are up. I have most clients using WAN1 normally, but use WAN2 if necessary. I also have other clients that use WAN2 exclusively.

  Now, instead of Link Backup, I use Policy Routing which allows me to force some clients to WAN2 only and the rest of the clients use WAN1 as priority and then the router will automatically route the majority of the clients to WAN2 if WAN1 is down.

  Don't know if this helps, but wanted to offer the suggestion in case this may work for you.

Website link: TL-R600VPN