I was using the Asus router for a while with the cameras... even found a screenshot with the config, hope it helps.
Things I remember:
- I created partitions with the right amount of space for each camera
- I created a specific user for the cameras and gave it access to the shares
- I specified Simpler share naming (without the disk name)
P.S. I think I also created directories with the same name as the share inside each partition.
(see the attached screenshot)