Unfortunately, there's no dedicated way to do what you're asking. What I could recommend is to make a script that always checks if Wired WAN got the link detected or not.
As you want it to check on "Connection established" you would also need to ping through that interface link.
To check if you got the link use third-party opkg package called ethtool. Download it with these commands:
opkg install ethtool
Once you're done with downloading, execute ethtool eth1 and you'll get the information about WAN port of the router, but what is important in there is "Link Detected" line. Use it to check if LINK is established.
If you want to check if you got the working connection through Wired WAN - you will have to ping outside networking through Wired WAN interface.
Use ping -I eth1 18.104.22.168 (or any other address that you like) and depending on the ping response - activate relay output with the script.