Some dongles are limited to 4K@30. Like this one. You’ll want to double-check the specs of your adapter. They do make DP/HDMI to USB-C cables for what it’s worth. USB-C is a frustrating spec because you can run a lot of different standards over it. Even with cables you’ll want to make sure they’re not running some older spec.
I just double-checked your monitor’s manual. It looks like HDMI 1 can only do 30hz. Only HDMI 2 and the DP ports support 60hz. Were you trying to connect through HDMI 1?