@Yo_D1 I was under the impression that unless they have been disabled the root and admin local users should always function even when remote authentication is responding or down. This article could be related to your issue.
/config not mounted cause that error.
Use "mount -o remount,rw /dev/mapper/vg--db--vda-set.1.xxxx /sysroot/xxx " --"chroot /sysroot"
Then "passwd root" is ok. But after reset password and login , still failed.
root can login from local.
Seems reset the root password successfully , but still can't login from console.
I'm trying figure out
Saw your logged in screenshot, but didnt realize you were in single user mode. My bad! Yeah, I dont think all of the correct mounting is done at that point to support TMSH commands. As @Paulius suggested, this final target link (https://my.f5.com/manage/s/article/K35811337) may be what you are after here.