Inspiron 5676, Radeon RX 580, U2417H's go dark

I have two U2417H monitors connected via mDP to an Inspiron 5676 with AMD Radeon RX 580. I only recently bought the Inspiron 5676 and U2417H. Over the last week or so, the U2417H's will 'go to sleep' (or go dark), sometimes even when I've only been inactive a few moments. I don't believe this was happening when I first connected the system up. It seems to be a new issue. Is this some power saving function? I have Windows 10 set to not allow the monitor to sleep or go the screensaver for 20 minutes. Can anyone give me some ideas what I can do?

Thanks in advance.

D