If ghusl is wajib on an individual and that individual is not able to perform ghusl before subah sadiq (the break of dawn), if they eat suhoor and commence their fast before performing ghusl, their fast is deemed valid. In essence, being in the state of janabah while having suhoor does not affect one’s fast.
That being said, the individual should perform ghusl as early as possible. Delaying ghusl to the extent that the fajr prayer is missed is a major sin.