XPS 15 - 9570 Not recognizing Monitor

Are you sure you're using an actual Thunderbolt 3 cable rather than a USB-C cable?  They use the same physical connector, but there are wide range of USB-C cables when it comes to capabilities, and few if any of them will actually handle Thunderbolt 3, whereas true Thunderbolt 3 cables will work for Thunderbolt 3 (assuming they're not defective or anything) and actually in some cases will NOT work as full-featured USB-C cables.

If that's not the issue, here are few other ideas:

Make sure you have installed the latest versions of the following, in this order:
- System BIOS/firmware
- Thunderbolt 3 driver/software
- System Thunderbolt 3 controller (not part of the BIOS update)
- Intel Graphics driver

Also make sure Thunderbolt is enabled in the BIOS Settings.  You don't need boot support enabled, but as a test you might want to try temporarily switching the Thunderbolt security level to "No Security" rather than the default, although I wouldn't recommend keeping the system in that state because unlike USB, Thunderbolt grants direct access to your system's memory, which means without security enabled, a malicious device could potentially pull sensitive data out of your system simply by being connected.

If none of that works, are you at least getting the prompt to "approve" a newly connected Thunderbolt device when you connect the display?  This prompt would be bypassed while the port is set to No Security, but you should have been seeing it thus far, and until you approve the display it won't be recognized as a display.  If you go into the Thunderbolt software and select "Manage Approved Devices", is there anything there?