Take the long view.
If you are at the beginning of a new chess career that will include many events - this is just a start-up bobble. If you had, in fact, never been rated - you would be in the same situation.
Now, the TD could look at your recent results and assign you a rating for pairing and prize purposes, but he is within his rights to refuse to do that. There are very good reasons why UNR players are restricted to either the Open or UNR section in many events. Too many players have manipulated the system. Perhaps this TD has been burned in the past.
Now…if you really are an 1800+ player, then playing in the Open section would be a good choice. You’ll get walloped, but at least you won’t get the Bye!
If you really are a 1300 player, then playing with the kids can be an enjoyable experience. Take it as an opportunity to demonstrate to the kids how to behave (and, hopefully… how to win).
Either choice is at least tolerable. And…at your next event, you will have a published (if still Provisional) rating and will be able to play within your class.
if that’s what you want. For what it’s worth, my standard advice is:
a) if your published rating is within 100 points of the top of your section, you should “play up”
b) if your published rating is within 200 points…, you should consider playing up.
c) if your published rating is greater than 200 points from the top…that’s the right section for you.
Note that you won’t win much prize money as (say) a 1798 player playing in the U1800 section. All the prizes will be won by fast-improving 1650 players.
(see the crosstable for my tournament last weekend…)