Enter data yourself?
By creating pages and a data structure (creating entities and attributes) that does not create data.
I assume you have configured the add buttons on all overview pages. There you connected an entity and a page. This button will create a new object of the connected entity and show the page, containing that object. use your imagination to add demo data to it.
Please check the user roles access for the entity ( if you any user roles ), from the which the data is fetched to the overview pages, and also check whether the microflows of the overview are called properly from the pages where they are needed. Or just try to add new data by yourself and check whether it gets added to the database in console->Advanced->start-builtin-database viewer and select entity module to which you added the data and execute the sql query. If data exists there then you’re not displaying and if the data is not there you’re adding the data wrongly.
cheers and good luck!