1) Go to the AiDisk configuration page on your router, add a new user, and give that user access to the folder you wish to share.
2) Network shares on the router work at the folder level, not at the whole disk level. If that is an issue, then create a folder at the root, share that folder, and move all subfolders inside that new folder.
Asuswrt-Merlin: Customized firmware for Asus routers
Github: github.com/RMerl - Twitter: RMerlinDev
See the sticky post for more info.