Praise be to Allah.
I do not find another solution to this issue, except to request from the prison administration that the time be changed to between maghrib and fajr. This is the time where it is permissible for a man to have sex with his wife. Whatever be the concerns taken into consideration, it is without doubt impermissible for a Muslim, who is obligated to fast to have sex with his wife during the day time of Ramadhan.
This is one of the worst things that break the fast and upon committing this act, it is obligatory for the person to free a slave. If this is not possible, then fasting two months consecutively. If they are unable to do that, then feeding 60 poor persons, repenting, and fasting the day one had sex on.
It is upon you all to give instruction and advice in a good way. If something happens then repentance and expiation are required. It is hoped that the administrators of the prison will respond to the mentioned suggestions, particularly if they are able to provide individual rights and religious freedoms.
May Allah facilitate for you every good and reward you for your struggle. We ask Allah to give firmness to the Muslim brothers who are in prison and to guide their wives. Allah is the guider to the straight path.