How to get 60Hz on 2x 4k TVs

Hello, I have an iMac Pro and 2 4K TVs that I'd like to use as additional displays. It says on the iMac Pro spec page:

• Two 5120‑by‑2880 (5K) external displays at 60Hz with support for 1 billion colors, or
• Four 3840-by-2160 (4K UHD) external displays at 60Hz with support for 1 billion colors, or
• Four 4096‑by‑2304 (4K) external displays at 60Hz with support for millions of colors

I connected 2 4K TVs with the Apple USB-C Digital AV Multiport Adapter, but am only getting 30 Hz and there is a noticeable delay. These are the TVs I'm using:

Amazon.com: Samsung Electronics UN43MU6300 43-Inch 4K Ultra HD Smart LED TV (2017 Model): Electronics

Is there a setting somewhere I can change to get 60Hz? Do I need a different adapter or cable? Or do I need to get different TVs? Also, is there a way to turn off the display on the iMac since I'm only using the TVs, or is it stuck on permanently?

Thank you for any help.