Namespace "ExitGames" not found

I'm trying to use the latest Photon .NET SDK version from the website ( in my C# project.
I added Photon3DotNet.dll and PhotonLoadbalancingApi.dll from the libs/Debug folder of the SDK to my main game project.

But it keeps telling me that "Namespace 'ExitGames' does not exist" when I try to "using ExitGames.Client.Photon". I checked the demo examples (the load balancing demo) and the same 2 DLLs are also referenced there. The namespace gets recognized without any problem in the demo project (The demo project also references Unity Engine, but I don't think that the Namespace is in there because there's no Unity-DLL in the output directory of the demo project).

The demo project compiles and I can open a room (some buttons don't do anything, but I don't know how it should be, so maybe that's expected?).

I wonder if any of you has an idea as to why my namespace is not recognized at all in my main project.
The main game project is created from the MonoGame Windows template. It runs on .NET 4.5.

I have multiple projects in my solution and when I tried one that was a normal WinForms application running on the same .NET version, the namespaces got recognized instantly.
Therefore, I am guessing that it has to do with the MonoGame template. Are there any project settings that could prevent the namespaces from being recognized? Or something else that could be affected?

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