When you have a user from that domain try to log in, and they can't, do you then see the user under the 'users and access and roles' section? with like 'non selected' as a role? If so, set the role there?
Also, have you added them to a group on your original domain - like 'Lansweeper Admins' or something, and added the AD group in Lansweeper (under 'User Roles' > choose 'administrator' group > click the globe icon to 'add group' > add that group you made)?