2 pages of speculation and the OP didn’t even describe who they have the issue with.
Not enough info given to know what to reccomend.
Was the dealer to register it?
Did the buyer get a MCO and thinks a title will magically appear?
What did the buyer do to register it? If anything yet?
Was the trailer new or used, sounds like new.
Is there a loan on it? Does the bank have it?
Need more info.
But new trailer, paid in full, either the dealer sends in title and reg or buyer walks away with a mco and does it themselves.