Have you tried setting the mount point name to “/mnt/MyPassport” with no blanks? The Mount point name can be different from the Label on the USB drive. Also, turn on “Enabled” in the LuCI settings.

It is safer to avoid blanks in general, which is why I suggested “/mnt/USBDRV” above, instead of “/mnt/USB DRV”.

For the EXT4 format on your USB drive, check that the kmod-fs-ext4 package is installed.