Calling functions between 2 classes
There is a fundamental difference between a simple function and a function that is a member of a class.  You can't use a function pointer to point to a member function of a class: simply because that member function also includes information about the class instance that it belongs to.
The proper C++ way to handle it is to use std::function, but the Arduino AVR compiler lacks the full STL (Standard Template Library), so that can't be used on normal Arduinos. Other cores do have it, so you may well be able to use it on the ESP8266.
If you want to make your code portable between systems that may not have the full STL then there's other tricks you have to do:

Use a wrapper function (which as you have found works), or
Make a polymorphic class and store a pointer to the instance cast to the parent class - the parent class defines the functions that are allowed to be called from the pointer.