OData unbound function opt-in

I have a controller which contains certain kinds of methods. This controller is housed in a separate assembly. The idea being that other APIs can then opt in if they want to use this functionality as unbound functions.

The problem is that if I don't implement this functionality with the ODataConventionModelBuilder OData returns the following error:

System.AggregateException
  HResult=0x80131500
  Message=One or more errors occurred.
  Source=<Cannot evaluate the exception source>
  StackTrace:
<Cannot evaluate the exception stack trace>

Inner Exception 1:
InvalidOperationException: The path template 'TestFunction' on the action 'TestFunction' in controller 'OptInFunctions' is not a valid OData path template. Resource not found for the segment 'TestFunction'

If I remove the ODataRouteAttribute from the functions then OData does not return an error during startup, but a 404 when I want to call the function.

My controller with the opt-in methods looks something like this:

public class OptInFunctions : ODataController
{
    [HttpGet]
    [ODataRoute("TestFunction")]
    public ActionResult<bool> TestFunction()
    {
        return Ok(true);
    }
}

How can I make an OData unbound function opt-in?