When the SeapHOx system loses connection with its 37 SMP-ODO CTD component, the system will report back the measured AND derived values related to that instrument as NAN. As part of this, it will also report its External pH as NaN as the salinity correction is missing the CTD's input. The external pH requires the CTD's temperature and salinity input in order to make a valid measurement.
The CTD data- the temperature, pressure, oxygen, and salinity come through this Y cable connection. Check to see if your CTD has lost power (either external SeaFET power or internal lithium AA cells) and stopped transmitting data to the SeaFET. If both the CTD and SeaFET are working the next most likely point of failure is the connection between the two units, primarily due to either an incorrect setup of the communication settings or due to a physical loss of connection.
Inspect your connection for signs of a bad cable, bent or loose pins on your bulkhead connector, or water ingress. For the settings side, refer to the manual and integration guide for the correct settings.