بسم الله الرحمن الرحيم
(Fatwa: 117/73/H=1/1438)
There is no such ruling that if you missed your Fajr salah then your Isha was not valid. In short, your Isha salah was valid in the above mentioned case.
Allah knows Best!
Darul Ifta,
Darul Uloom Deoband
