If pressed to guess, it sounds like you are trying to use a USB-C Alternate Mode video adapter to connect an external display to the laptop’s USB-C port.
Assuming fro the moment that my understanding is correct, if I recall correctly Purism has stated in the past that the USB-C port in the Librem laptop is for data use only. It does not provide USB-C Alternate mode video output, nor can it accept power from a USB-C Power Delivery charger or dock to charge the system.
I tried to find previous postings supporting my recollection, however I could not seem to find them today. Maybe someone from Purism could offer a definitive answer (or confirmation).