External monitor USB-C with DisplayPort to HDMI works initially and then stops

I have the following setup:-

1. DELL Inspiron 17 5000 series laptop running Windows 10 with
   • 1 x HDMI port
   • 1 x USB-C DisplayPort port
2. 2 x External Acer Monitors each with
   • 1 x HDMI port
   • 1 x DVI port
   • 1 x VGA port

Originally I had only a single external monitor connected by HDMI. I recently got a second external monitor and thought that I could connect it using an HDMI to USB-C cable since my laptop has the DisplayPort symbol next to the USB-C port. Note, I use my laptop with the lid open, so this means I am trying to move from two screens to three screens.

This worked beautifully the first time it was plugged in - gotta love plug and play! I used this for an hour or two, and started getting used to having particular applications on that third screen.

After some inactivity, my laptop power settings are such that the screen will turn off after 15 minutes. So later when I came back to my laptop after the screen had turned off (and both the external monitors), then the new second external monitor now no longer gets a signal. I repeat, it worked earlier.

There is a device called "USB 2.0 BILLBOARD" that appears to be what the laptop recognises as the HDMI to USB-C DP cable.

I have tried ejecting and reconnecting the USB cable. I have tried rebooting the machine and plugging in the cable before startup and after startup. I don't know what configuration might be necessary since it didn't need any the first time when it worked.

All suggestions, or clarifying questions gratefully received.