When restoring a database, mapping the unc directly did not work for me. I got a permission error.

I created a share where the backup file exists, then used the unc directly to the share:

Example:

so instead of

\\111.111.111.111\f$\SQL\BACKUPDIR

I created a share called BCKSHARE on SQL\BACKUPDIR

Then I used

\\111.111.111.11\sharename

This worked for me.