Official troubleshoo

Dell 2720Q, DELL2720QM¹, a 4K monitor released in 2019.

You want: 4K and 60Hz. You can check what you currently are on

 > About this Mac > Graphics/Displays

How to configure

1. On Dell U2720U Monitor’s menu (the button before the power button):

Display > Smart HDR > DisplayHDR
        > USB-C Prioritization > High Resolution

2.  > System Preferences > Display > (Hold alt/option key) select Scaled, then you can select resolution you want. You should also see your refreshing rate.

If you cannot make it to 60Hz, reset monitor to factory setting, unplug the cable, try to configure again!

It is ridiculous a few thousand dollar laptop and $800 cannot just work.

Good luck!

P.S. watch YouTube, use Chrome. Watch Netflix, use Safari.