Please read the "Using a DMR connector with a C4FM radio" section in the user manual at
http://manuals.sharkrf.com/openspot2/en/cross-modem-modes.html#page-top
Also the "Cross mode usage" section at
http://manuals.sharkrf.com/openspot2/en/homebrew-mmdvm-connector.html
Make sure you've set the "Route cross mode calls to ID" (Homebrew connector settings) to the talkgroup you want to talk on, and set the call type to group call.