XPS 15 9570 With External Monitor - Sleep Issue

Hey community, 

I'm having trouble with using my Dell XPS with an external monitor. The monitor shuts off after about 30-60 seconds of no use, even though the power management settings are set to either "sleep after 5 minutes" when on battery or "never sleep" when plugged in. The laptop behaves normally and respects the power management settings when I'm not using an external monitor (or when I'm using both the laptop screen and the monitor simultaneously). It is only when I close the lid of the laptop and use just the external monitor does it stop respecting the settings I have set. I don't suspect it's the monitor because the monitor itself has no sleep settings and my other accessories connected to the PC also shut off when the monitor does. What would be causing this? 

Here's my hardware: 

Dell XPS 9570

Asus VG278QR Monitor 

USB-C to DisplayPort cable

Thanks