Have you ever gotten DNC to work with this control at all? Does loading into memory work?
What is your procedure when trying to DNC? Typically you put the control in TAPE or DNC mode, press Cycle Start, then send from the PC. Also, 300 baud seems awful slow, especially for DNC'ing.
I'd suggest proving out basic communication before trying to DNC to it. Get it to communicate both into the control and back to the PC. You need both directions working correctly in-order for DNC to work.