U2723QE, MacBook Pro 2017, 4K 30Hz?

So I had this exact same issue when I unboxed my Dell U2723QE this morning, and tried to connect it to my Macbook 2019 Pro. (using the USB-C - USB-C cable provided - which actually plugs into the DisplayPort 1.4 90W port in the back of the monitor)

Here's how to get it up to 60hz:

1. Follow what Chris M posted above:

* Open the U2723QX/U2723QE/U3223QE Menu- Display
* Go to USB-C Prioritization
* Verify it is set to High Resolution

2. Plug in your cable to your Macbook. I've read things online about how you need to plug it in to the left-side vs. the right side. (when looking at the screen) This doesn't matter haha.
3. Search for "Displays' in spotlight search (CMD+space) or go to System Preferences in the macbook launchpad and go to the 'Displays' settings. You should see 2 windows. The main window that's for your Mac and the other one for your Dell monitor.

4. The Dell Monitor resolution should be initially set to default for display. Switch it over to 'scaled'. Now you'll see a range of options similar to your main window going from "larger text" to "smaller text". Now hold down the option key on your macbook and click the "scaled" radio. Yes, I know we've already selected it. Now you should see the range conver to a list of resolutions. Click the checkbox called "show low-resolution modes". Now you'll see a refresh rate dropdown appear - and it shouldn't be greyed out or disabled. It'll be set to 30Hz. Pick 60Hz.

* If the refresh rate dropdown is greyed out, you either have a bad cable (don't see how because Dell gives you one with their monitor) or you didn't do step 1 correctly.

Hope this helps!

I recently purchased the U2723QE and attached it to my 13" MBP 2020. I followed yashsway's advice and that definitely worked, but only in low-res mode. After trial and error, the following steps enabled the 60Hz option in the high-res mode:

The first few steps are the same as Chris M and yashsway:

1. Open the U2723QX/U2723QE/U3223QE Menu - Display
2. Go to USB-C Prioritization
3. Verify it is set to High Resolution

Then try the following steps-

1. Open the U2723QX/U2723QE/U3223QE Menu - USB section
2. Go to the "DP assign to" option
3. Choose the "USB-C1 -> 90W" setting
4. Plug in your Mac using the provided USB-C to USB-C cable, with the other end attached to the USB-C1 90W port on the monitor.
5. Then, on your MBP, go to System Preferences -> Displays settings page. 
6. Click on the Dell monitor window to see its settings pane.
7. Choose any desired resolution (to see the complete resolution list follow step-4 of yashsway's response).
8. You should now see the 60Hz option available for the refresh rate. Pick 60Hz.

These steps should enable the 60Hz mode for high-res options, as well. 

Btw, you could also download the Dell Display and Peripheral Manager Application to easily change monitor settings without the need to twiddle with the joystick behind.

Hope this helps!