Documenting here the answer that I gave to you on the other thread:
For the mvebu WRTxxxACx routers, you need changes to to /etc/config/system and /etc/config/network.
You shouldn't sysupgrade with those files in place. (and you can't restore them from old backups)
Ps.
The recommendation is to sysupgrade without settings, but a quick-and-dirty approach is to remove /etc/config/network and /etc/config/system before sysupgrade and keep the other settings. Those two will be re-generated at the first boot with the new firmware, so you get the new defaults. (E.g. renaming those to network.old and system.old before sysupgrade is sufficient. That has worked nicely for me with R7800 kernel 5.10 DSA testing)