I tried all possible ways but none seem to work and this is causing the whole project to go flat. Immediate suggestion would help, not very familiar with class function pointers. I know I shouldn't have used it without much knowledge but right now there is no going back so help will be appreciated.
In general, don't keep a reference to member functions; keep a reference to the instance instead. You cannot invoke a member function without the instance it needs to invoke on (because the this context would be meaningless). Keeping a reference to the instance not only makes this easier but also make your code readable and easier to understand for others.