Thanks to your help I now have audio passing between the RCC's (with no data connection to a radio), using Mode 4.
I'm having a problem getting PTT to pass to the RCC Radio AUX/MIC connector pin called PTT.
Grounding the PTT AUX/MIC pin at the RCC Control correctly toggles PTT in the RCC Radio STATUS page to ON. It also stops audio coming from the Radio which I assume is correct (maybe there's a setting to stop that happening). However it does not toggle the RCC Radio PTT pin; in fact that pin on the RCC Radio is always grounded. Am I missing a setting that enables the RRC Radio PTT pin on the AUX/MIC connector?