Unity "Type or namespace could not be found" with external NuGet package, no error in Visual Studios

I installed a NuGet package through Visual Studio into my project. In Visual studio I can reference the namespace that was part of the package, use it, intellicode works fine, no errors. But in the Unity console I get t$$anonymous$$s error:

The type or namespace name '' could not be found (are you missing a using directive or an assembly reference?)

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I saw some similar issue but the solutions seemed to be to add the dll to the asset folder. But I don't have the dll to put there, it's a NuGet package.

Any help or advice is appreciated. I can give more information if necessary I just don't know what is relevant. First time even using NuGet or external references like t$$anonymous$$s.