What to do in case of ignorance of the termination notice of the tenancy contract?

Dear sir/madam,

I was not aware of the 90 days prior information to be provided to the landlord and neither the agency told me about it while signing the contract.

It is nowhere mentioned on the contract that I should notify 90 days prior to the end of the contract term. I did inform them 26 days before vacating and then the agency told me that I should have informed them 90 days prior and now there will be a penalty.

I feel cheated through the lack of information provided. What to do in this case?