Hmmm, I dunno why it doesn't work then.
I assume you see "Mounted '/tmp/mnt/drivename/entware' on /tmp/mnt/drivename/asusware.arm" instead of "Mounted '/tmp/mnt/drivename/entware' on /tmp/opt" in the syslog?
Maybe try rebooting the router if it is not a problem, unless you already did that earlier.