Just posting this to help anyone who had the same problem I did.
I was doing a boygrouping setup where the server was a single monitor laptop whilst the client was a dual monitor desktop I wanted to run in horizontal span.
The problem was that every time I put the client into fullscreen it would only be one screen, not across both. This was due to the resolution selected via one of the renderer ‘hidden pins’. However as it’s an enum thay you can only change from the inspektor I could only select resolutions available on my Server (1024 x 768) rather than the one I needed on the client (2048 x 768). It was impossible to use the client ID to choose a different value on the client because there is no pin for it.
The solution I found after much searching was to use MSBERGER’s SetRes module. link