When you connect a new display to the CMS, it will show you a message that it was added to the CMS, but need to be authorised by the administrator.
Which in this case means by you, please log in to your CMS, navigate to Displays page -> Edit your display -> Change' Authorise Display?' to Yes.
Here you can find the instructions with screenshots https://community.xibo.org.uk/t/installation-guide/4326
The guide is for android, but it's the same logic for windows client in the CMS itself.
The screenshot in this guide is from 1.7 series, so it's says 'Licence display?' instead of 'Authorise display', but functionally it's the same.