The last released version of software for the 1198c Si unit is 5.700
Sorry, can’t upload it to this web site as the file is too large.
Call the Humminbird Customer Resource folks about not being able to register your unit.
If needed I can send you a link to TransferBigFiles.com so that you can download the 5.700 software.
Did you try swapping which unit is connected to which port? All you have to do is swap over the connecting cables at the InterLink.