Hi!
I'm understand what i'm doing a little
My router connects to ISP through another switch, then to GPON device (pri wan). Secondary WAN is just another router with LTE modem (mikrotik).
When i want to simulate Pri wan down, i just disconnects GPON device from switch, so AC-68U didn't loose link, but default gateway of pri wan from ISP becomes unreachable. It switches to secondary wan correctly. When default gateway becomes reachable again during 4 pings on each 4 seconds, it switches back.
Yes, of course, this schematics has disadvantages - only one target, no speedtest, no URL check (for example DNS or ICMP working but if your ISP blocks you access to internet because of billing - then this will not work).
But on previous version of FW, even this simple schematics didnt work - when router trys to failback it became switching between pri and sec wan and vice versa.