Are you using integrated or external graphics? With an external GPU, you'll need to connect the DisplayPort of your GPU into the DisplayPort-IN 1 of the motherboard and then use USB-C 1 to connect your monitor. Otherwise, with integrated graphics, USB-C 1 is supposed to output the DP signal by default. If you're changing around cables, make sure to do it when the system is off.
I don't have this MB and don't use thunderbolt graphics on my system, but hope this is helpful!