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To make things simpler, I would start by turning server side off. It’s not related to an ajax datasource and requires you to return extra parameters. For example, you need to return the number of total records and the number of filtered records. I didn’t see that in your code. See the documentation: https://datatables.net/examples/server_side/. Because you’ve omitted those params, that could be why you are not seeing anything.
Edit: Look at the server side processing docs: https://datatables.net/manual/server-side#Sent-parameters. Paging works by sending a start and a length in the sent paramters. You obviously need to use that information to query for your data.
As for server side filtering, you need to set the records total and records filtered.