The method/function that you try to set as callback - in your case CellLocate::_cbLOCATE.
Simplified background:
Static methods "are part" of the class and not of the instances, and as such can only directly access static variables of the class, since they have no means to resolve which instances set of variables to use otherwise.
That is also the reason why instance methods can't work (unless there was a dedicated overload of CellularClass::command() that took an instance method plus instance pointer).
Non-static methods could try to access instance variables which is not possible since CellularClass::command() cannot resolve the respective set of instance variables, but for static methods it's ensured that this won't be an issue.
These are some threads that deal with the same problem (but other context)
How can we use Particle.function from within a class?
How to use software timer within a class?
How to use Timer inside a C++ class
Using Timer in a class