I see you are right about 19.07.10's recent release. However, my source that i was looking at ...
downloads.openwrt.org
...lists that version four down in the list of archived releases, with the caveat "The releases below are mostly for historical interest - they're generally out of date, and no longer maintained."
This might be useful to you:
According to Table of Hardware
...your router supports 21.02.3.
My router is completely unusable with 21.02.3 due to lack of memory in router. Theory is theory, practice is practice. Oom-killer kills the processess and router reboots all the time.
That's too bad. Time to buy a new router, I guess.
In theory, theory and practice are the same. In practice, they're different. -Yogi Berra.
I have the one. But is has some issue with OpenWrt (No configuration from ISP on Asus RT-AC85P - #12 by bill888). Not luck.
With recent updated from OpenSSH, I no longer can access Openwrt with the same error as above. After google around I now can access Openwrt with following option:
ssh -oHostKeyAlgorithms=+ssh-rsa -oPubkeyAcceptedAlgorithms=+ssh-rsa -i /config/ssh/id_rsa root@192.168.1.1