Apple - External display works on 30Hz

I had the same issue for a long time. At first, 60Hz worked. Then something happened and I'd only get 30Hz on my 4K Lenovo ThinkVision 27".

I stumbled upon this article and simply went to Display settings and hit "Resolution: Default for display". Immediately the display switched to 60Hz. Then I could reapply my Scaled text size setting and 60Hz remained.

What an unsatisfying fix!

The Mac should automatically set itself to 60 Hz when available. Otherwise you can select 60 Hz from the drop-down titled "Refresh Rate".

In your screen shot that drop-down is greyed down, because it only has one option (30 Hz). This means that the Mac has not detected that the cable and monitor are cable of 60 Hz refresh rate.

As the monitor supports 60 Hz and your Mac does so as well, all points to the cable. Please try again with another cable, and make sure you buy a quality cable that supports bandwidth enough for 4k @ 60 Hz.

I'm here to offer a different solution. I have a MacBook Pro 15(2018) connecting with LG Ul500 27'4k monitor. I met the same issue that the external monitor worked on 60hz at the beginning but changed to 30hz one day. I thought there might be something wrong with my cable, so I changed it with one of higher quality and price. But the situation didn't change. Then here comes the solution.

I enter the menu of my monitor and tried to reset it to the initial settings. Boom. Problem fixed. It's working on 60hz right now.