Problem with external 4K Screen

Dear community,

I'm writing this post because I'm in trouble with my XPS 15 9560.

I've recently bought an external 4K Screen (Lenovo L28U-30) and since then I'm experiencing some issues. 

When I connected my new screen, everything went slower, I often work simultaneously with heavy programmes such as Revit, Photoshop, InDesign, AutoCAD and there were no problems before connecting with the external monitor. 

Then, I installed its driver, I updated all of my drivers and the Windows version (now it's 20H2) and the BIOS as well. Unfortunately, the problem is still here.
This monitor is a 60 Hz screen and it works only with a Refresh rate of 30.011 Hz.

So my questions are:
Do you guys know how can I do to allow the change between 30 and 60 Hz?
This laptop works with two graphic cards, Intel Graphic card and GeForce, did any of you guys find the solution for switching from one another?

Thank you very much for your help and support.

Antonio