@Clemence45 I cannot tell from your profile where you live, but it is very possible that it is not the landlord that is restricting the number of nights that you can sublet on AirBnB, but the municipality that you live in.
In either case, if the landlord has said 90 nights, then it would be best to do as they ask. Once you have established yourself as a good host, the landlord may allow more nights (if permitted by law).