Problem with orderby on JobRegister query

I use the .NET SDK in a custom reporting application.  The client using it makes extensive use of Jobs in the MYOB usage, and there are a small number of top-level jobs that represent their operational departments.  Most of these have a number of sub-Jobs down to 2 or 3 levels deep. Every transaction is assigned to a Job.  I recently had to make a query to the JobRegister endpoint, and I wanted to order the results by Account/DisplayID and then Job/Number.  My filter string looks like this:

$filter=Year eq 2024 and Activity ne 0&$orderby=Account/DisplayID,Job/Number

This is rejected by MYOB with Bad Request.  I have not been able to come up with a syntax that MYOB will accept, so I am obviously missing something here.  If I remove the orderby part, it works fine.  I have used orderby in many other queries, but I have not had to specify the field in a nested way as above.  So, what am I doing wrong here?  Thanks.