This error message, "Unknown package 'nfs-kernel-server'," indicates that the opkg package manager on your GL-AXT1800 router cannot find the nfs-kernel-server package in its configured repositories. This can happen for several reasons after a firmware update:
Repository Changes: Official Site
The firmware update might have changed the repositories that opkg uses.
The updated repositories might not include the nfs-kernel-server package.
The repository URLs might have changed, or the repositories might be temporarily unavailable.
Package Removal:
The new firmware version might have intentionally removed the nfs-kernel-server package from its repository.
It is possible that the package is in a different repository that is not activated by default.
Repository Index Issues:
The opkg package manager might have an outdated or corrupted index of available packages.
This can happen if the update process was interrupted or if there were network issues during the update.