Did you update your Bios? if not, you should.
Just built my new 9600k yesterday with a MSI Gaming Plus Z390, updated bios, using Corsar H100i V1. I've my voltage in Auto in Bios and set it to 5.0Ghz. No problems at all, temperatures around 60 - 64. I think that there is room to go to 5.2 Ghz...