Make sure the Half Deviation setting is turned off in your transceiver, and use plain C4FM modem mode on the openSPOT. If there's no such setting in your transceiver's menu, then you have to use C4FM Half Deviation modem mode, and run C4FM Half Deviation AutoCal on the openSPOT (or manually change the RX offset on the Modem page for the lowest BER). Test the connection between your transceiver and the openSPOT using the built-in local echo service.
http://manuals.sharkrf.com/openspot3/en/control-calls.html#local-echo-service