I followed the instructions on this page https://openwrt.org/toh/asus/rt-ac57u
As there were no install file, I used the upgrade file.
Restarted the router after install
You aren't really specific from what you upgraded either (18.07.x, 19.07.x, 21.02.0 or OEM), but if I had to guess -especially considering the mt7621 target-, I'd put my money on an 19.07.x (or earlier) to 21.02.x upgrade, which means migrating to DSA (there should have been a huge warning message with further details). To cut the story short, and before going into further details at this point - give https://openwrt.org/docs/guide-user/troubleshooting/failsafe_and_factory_reset a try, in the form of letting your router fully boot up (wait at least 2 minutes after powering up, better 3-4 minutes to be on the extra safe side), press the reset button for over 10s (again, a little longer -like 20s- won't hurt) - leave the router alone for at least 5 minutes and check if you can get access again (first via your browser, if that fails via ssh).
Performed failsafe reset and after 5 mins nmap 192.168.1.1 stated that the host is down
however -Pn shows that the host is up
Starting Nmap 7.92 ( https://nmap.org ) at 2022-01-23 15:05 UTC
Nmap scan report for ASUS_router (192.168.1.1)
Host is up (0.072s latency).
All 1000 scanned ports on ASUS_router (192.168.1.1) are in ignored states.
Not shown: 990 filtered tcp ports (no-response), 10 filtered tcp ports (host-unreach)
Nmap done: 1 IP address (1 host up) scanned in 5.92 seconds