Reference class in normal c# script from within a (soon to be) dll script?

I'm about to compile my first dll script, but I have one line of code with a problem: It references a class that I want to maintain outside of the dll, as a dependency (if possible).

I don't want to make that class a part of the dll because it's just a FixedFrameCount class that uses the community's Singleton class (here) as basis, and can (and should) be common to other scripts, to avoid duplicate code to users who may already have either FixedFrameCount or the community's Singleton in their project(s).

I want to distribute those (`FixedFrameCount` and Singleton) as part of the asset (as dependencies), but not as part of the dll. But still be able to reference it inside the dll (currently giving me a "does not exist in the current context" error)...

How can I do this?

I am unsure if this will be useful in any way, but here is the relevant code:

The line of code I'm referencing FixedFrameCount at looks like this:

 case FrameDelayMode.FixedUpdate:
     FixedFrameCount.Instance.count + delay.frameDelay; break;

And here is the FixedFrameCount.cs's code:

 public class FixedFrameCount : Singleton<FixedFrameCount>
 {
     protected FixedFrameCount() { }//Prevent non-singleton instantiation
     public int count { get; private set; }
 
     void Start()
     {
         count = 0;
     }
 
     void FixedUpdate()
     {
         count ++;
     }
 }