MODBUS TCP Via SIM808 GPR

Hello there
I have a problem
I have ARDUINO MEGA and SIM808 EVB3.2.2 Module
There is an RS485 module
I can communicate with Arduino via modbus RTU and read data from modbus enabled devices. With the Modbus-regbank library.
But I cannot connect to GPRS data via MODBUS TCP.
Can you help with this?