If your MT3021 supports Modbus (check Section 7 of the manual), you will need to set a unique Node ID (1-247) and baud rate (9600, 19200, 38400) via software registers rather than DIP switches.
After entering the values, the manual advises to and then run a verification test (Section 6.5). mt3021 manual
Capable of programmable speeds and acceleration (up to 500 steps per second) to respond quickly to cooling demand changes. MT3021 Hardware User Guide (Reference Piece)
Verify that the "Output" LED is lit. If it is on but no power is flowing, the internal fuse or relay may be damaged. Enter calibration mode (Hold + Power On)