Have you tried the methods in this thread
https://slickdeals.net/f/9330575-as...dual-band-gigabit-router-59-free-shipping?v=1
I have 1 TM-AC1900 running John's fork for more than a year
Thanks. I followed pretty much the same instructions and get my router flashed to AC68P successfully. Problem came up when I tried to flash latest Merlin. Some kind of protection Asus put in the fw forced the router to revert to TM firmware. And the protection is apparently against any further firmware downgrading or upgrading...