Hi @bto32
Here are two links that show how to remove the display assembly from the laptop so that access to the rear of the display panel is possible.
[https://topics-cdn.dell.com/pdf/xps-13-9350-laptop_service-manual_en-us.pdf|Dell 9350 service manual]. Scroll to p.55 to view the necessary pre-requisite steps and then the procedure to remove / replace the display assembly.
ifixit [guide|121591] guide.
Once you have access to the rear of the display panel, you can inspect it for any obvious damage. If nothing is obvious since it has failed the diagnostic test then most probably it has suffered internal damage and needs to be replaced as it cannot be repaired.