Please perform a printer self test by pressing and holding the  FEED button and switching on the printer.  It will print the current printer parameter settings. It may be that the baud rate setting of your printer is not correct. 

The printer needs to be set to 9600 baud, which is the default setting. Please refer to the user manual. 
 
The correct printer settings are: 
Interface: RS232(EIA), USB 
Baud rate: 9600 
Data bit / Handshaking / Parity: 8 bit / RTS CTS / None 
Language status: English 
Printing font: 12 * 24 
Printing density: Medium 
Paper sensor setting: Medium 
Black mark mode: Off 
Cutter status: Without cutter 
Printing direction: Normal 
Command system: SPMP 
Default codepage: CP437