Need help as a newbie

Okay so I got an offer to work for an edtech start up (it's my 1st job) I really vibe with the company and it aligns well with my goals. I had a great conversation with the founder of the company too. During our conversation, I told them that I will be able to start from April 1st.

However, due to some unforeseen circumstances and things beyond my control, I'll be able to start only from May 1st. Now I am scared if they'll get angry due to change in the dates (I am overthinking this because I really want to work with them and am afraid to lose this opportunity)

I still haven't got my joining letter so I am thinking of waiting till they share it and then inform them about the change and request for remote work till I am able to join in person. Any input to this? Is this the right approach?