Mac Pro external displays problem

[GM Rod]

Hello!

 

I have been trying to connect my Mac Pro to my two monitors:

1x 27 inch UHD 4K Monitor | LG 27UD59P | LG UK
1x https://www.asus.com/uk/ROG-Republic-Of-Gamers/ROG-SWIFT-PG348Q/

 

Now, both have only one DisplayPort but they both also have HDMI.
I'm sharing these two monitors between the mac and my gaming PC, and the PC is using the DP inputs on both monitors.
I intend to connect the Mac Pro to a HDMI port on each monitor and be able to simply switch inputs on them, and voila, I have both computers on the same desk.

 

The cables I'm using are:

1x HDMI - HDMI cable

1x MiniDisplayPort - HDMI cable

 

The issue is, the HDMI-HDMI cable works on either monitor, connected to the Mac Pro's full size HDMI port.
But the other cable won't work. On the LG monitor, nothing shows up. On the ASUS one, I see a weird green blotch on top of what should be the desktop, and it blinks on and off.

Now the reason I'm thinking about is the refresh rate.
Now, the LG page says this monitor is capable of 56Hz ~ 61Hz Vertical refresh rate.

The ASUS one has different supported refresh rates depending on the input, so for HDMI it's 24Hz ~ 60Hz.

 

Since the Mac Pro works with BOTH of them if I just use the HDMI cable, I'm assuming the Mac Pro is sending out a signal that's somewhere in between the two monitors' supported refresh rate.

Which means the second input, via thunderbolt port should also work.

But it's not working. I've tried to connect it to both monitors, both by itself and with the HDMI cable also connected.

When I have both cables connected to both monitors, the Mac does detect a second monitor and extends the desktop but I can't see anything on the second screen.

This leads me to think it might be the CABLE. I only have one of this cable here so I can't test this.

 

So I need to know if there's something else I could check and test before buying a new cable, AND if I do buy one, what do I look for to make sure I'm buying the correct cable?

 

Thanks in advance!