I just bought a USB-C portable display (ZenScreen Ink). It worked with all of my Beelink EQ12(s) and EQ12 Pro systems except for one.
It seems the problem is with the backend USB C port. USB C cable doesn't click into the USB C port. It does click in all the other of my Beelink (EQ12(s) and EQ12 Pro)
Also, this Beelink seems to have an older BIOS (Though I don't think that is a problem)
The BIOS for your machine is the latest, you do not have to update it.
But for your issue, USB C cable doesn't click into the USB C port, could you please send us corresponding photo to show this issue? or you can send us vedio? so that we can know your issue well.
Looking forward to your feedback so that we can help you better.
Have a nice day!