PC wont stay in 'Second Screen' mode, keeps jumping back to 'Duplicate'

[Larrens]

I have a Dell Latitude Laptop (Win10). I have been using this with a basic non-powered dual-HDMI hub plugged into the USB-C Thunderbolt port.

I have 2 external HDMI monitors with the laptop screen off (running in 'Second Screen' mode when I press Fn-F8)

I just got a Tagus Dock190 and want to continue using 'Second Screen' mode and leaving the laptop screen off.

I installed:

1. DisplayLink Manager (Application version 3.2.14.0 & driver version 11.2.3146.0)
2. Targus AV configuration utility.

My issue is the PC wont stay in 'Second Screen' mode.
Every time I set my main external screen as the "main display" in Windows settings, the press Fn-F8 and select second screen, it stays like this for a few seconds then jumps back to 'extend' mode i.e. the laptop screen turns back on and this then becomes my main display.

Any idea why this is happening - cou'ld it be that the installed software is conflicting, and if so which should I use - DisplayLink or Targus?

Thanks