After testing on another router, it seems SD/USB mounting has changed in the latest GL.iNet/OpenWRT firmware. It always mounts on /tmp/mountd even after I specified /mnt in /etc/config/fstab. As you have also experienced, I can manually mount in /mnt, but this is not retained after reboot.

Regardless of the mount point, at least Samba works and I still get permissions rwxrwxrwx for Everyone to have read/write access.,