Hi,
If you have an online account once the UTR is allocated the details would be shown there.
Once the UTR is allocated as the tax return will be issued to you late you will have 3 months and a week to submit the tax return before late filing penalties are charged.
As you registered after 5 October 2023 the original payment date, 31 January 2024, will apply.
If a balance is due and paid late you can appeal the penalty:
Check when to appeal a Self Assessment penalty for late filing or late payment
If you submitted your reference online you should have a submission reference that you can quote.
Thankyou.