Hi,
Just because the screen appears not to be damaged doesn't mean that it didn't suffer any internal damage that is not visible e.g broken connection in the display panel.
As a DIY repair all you can do is to temporarily replace the display assembly and check if it works or not. This is to find out if the problem is in the display or on the systemboard.
Here's the ifixit Samsung Galaxy A7 (2018) Display Assembly Replacement guide that will help.
Search online for Galaxy A7 (2018) display assembly to find suppliers that suit you best.
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