Cannot set date/time on my a75 chartplotter

[garybuyit]

I noticed my tide and current chart displays were not showing the same thing as the Navionics app on my phone. The time was way off, and I could not rely on the tide information.
So I went into System setup; System setting; Data Source; Time & Date; Select Time & Date Source. The screen then tells me "Source discovery in process" . . . but after a few seconds tells me "Date selection not possible", then "The following devices are preventing data source selection. E22158-Sea Talk-STNG-Converter S/N 0550260 Please see your user manual for details".
Well, there is no user manual for the STNG Converter. And I physically looked on the two Converters used in the system, neither of them has a serial number 0550260.
What do I do now?? Thanks for the help, in advance.