The prison here in the U.S.A. serves our suhoor meal precisely at the time when Fajr time has commenced.

Answer

In the circumstances you find yourself, eat the food and spend the day as if you are fasting. Whenever you find the opportunity, you should make qadha of the invalid fasts. Obviously you will find this opportunity only after your release. The fast will not be valid when eating at Fajr time. But since you have no option in prison, you will not be sinful. Nevertheless, recite Istighfaar, seek forgiveness and make dua for release. But after release you must fulfil these fasts.