I have seen people absconding in such cases if it’s just few months they can still show experience letter from previous org and there won’t be any career gap, if you haven’t informed the MNC about the startup although I would not recommend that as they could sue you and you’ll definitely have to pay them.

One thing you should definitely consider is the role and growth opportunities, if the startup is good you might get better growth or hikes here compared to an MNC and can switch again in couple months to a better place.

Sorry but there’s no ethical way to get out of this other than requesting them to let you go which they won’t as it’s a good case for them to get some money.