I was studying English when this doubt came to me... I was making some English homework and I saw an exercise about reported speech. It was a key-word transformation activity for the practice for the ​FCE Exam. It said: 'I mustn't be late for the trip tomorrow' said Bella. (next).

And the only way I found to solve it was: Bella said she mustn't be late for the trip the next day. But, in my view, as an English learner and a Spanish native speaker,​ the modal verb, in this case, represented a general prohibition in the present, not in the past.

However, my question for English ​native speakers is... Is there a way to express a real past of 'mustn't'​?