You dont have to pay any roaming charges for your data card.. I take mine with me all over India and I dont get charged for using the net out of Delhi. But its still a good idea to have a chat with reliance customer care.

It is very unlikely that reliance wont have coverage at Reis Magos but the best way to confirm would be to call up some one local over there and ask them if some one uses a reliance phone in the area and how the reception is

Hopefully some one from the area will read this and reply with some reliable info