Permissions Reset Failed/ Access denied

It sounds like you've only been playing with file permissions.  Have you looked at the network access controls?  If users can't get into the share at all, then you do need to check the network access as well.  Generally I recommend setting the file permissions to allow everyone access (both read and write), and manage access through the network access controls alone.

What happens if you run CMD on a PC, and manually enter the credentials?

net use * /delete /y
net use t: \\nas-ip-address\sharename /user:username userpassword

using the real values instead of the placeholders of course.  Start with admin, and the NAS admin password.

If you try this with multiple usernames, make sure you re-enter both commands, as you need to terminate any open SMB sessions before you change credentials.

Note I don't use AD, so I don't have any experience troubleshooting that.