Change the credentials type to String, then use c_str() to convert them to char.

// GPRSS credentials
String apn;
String gprsUser;
String gprsPass;
// MQTT credentials
String topic;
String broker;
String clientID;
String brokerUser;
String brokerPass;

Example:

modem.gprsConnect(apn.c_str(), gprsUser.c_str(), gprsPass.c_str())
mqtt.setServer(broker.c_str(), port)