As Robin2 said, just check what happens in PuTty with XON/Xoff disabled.
XON/XOFF is a handshaking protocol to ensure the Rx buffer at each end are not overwhelmed... With only 4 characters 44V (plus whatever message terminator you are using), is unlikely to overflow any buffer...
Chances are you can ignore it completely unless your messages are longer than several (e,g. 16 chars or so).
If the receiver sends long messages back to you, you may need to implement handshaking, but it seems the command are pretty short/terse.
Worst case for now, just send XON before each message - so gre receiver ‘knows’ it can reply to you.
Are you sure your Tx and Rx pair are correctly connected to the receiver?
One function I use when debugging is to ‘peek’ at the incoming characters - while specifically substituting markers for the non-printing characters,
... e.g . . etc
other characters pass through to Serial.print() so I can see the message.
This looks like an ASCII protocol, so the actual messages themselves are unlikely to contain non-printing chars.
This lets me see what the other end is sending back to me...