Compiler error: cannot convert function pointer to typedef function pointer

Question

I have to add further functions to an old project. This project have to use c++98.

If I try to add a callback function from within another class:

include <stdint.h>
#include <iostream>

typedef void (*cbFuncPtr)(uint32_t, uint32_t*, uint8_t);

namespace n1 {
namespace n2 {
    class lld
    {
    public:
        void reg(cbFuncPtr cb)
        {
            _cb = cb;
        };
        uint32_t exe()
        {
            uint32_t result;
            if(nullptr != _cb)
            {
                _cb(0, &result, 1);
            }
            return(result);
        };
    private:
        cbFuncPtr _cb;
    };
}
}

n1::n2::lld inst1;

namespace n1 {
    class lla
    {
    public:
        uint32_t getTime()
        {
            inst1.reg(&lla::callback);
            return inst1.exe();
        };
    private:
        void callback(uint32_t arg1, uint32_t* res, uint8_t arg2)
        {
            *res = arg1 + arg2;
        };
    };
}

using namespace std;

int main()
{
    cout<<"Start"<<"\n";
    
    n1::lla myTime;
    
    cout<<"Result: "<< myTime.getTime() << "\n";
    
    cout<<"Finish"<<"\n";
    return 0;
}

I'm getting the following compiler error:

error: cannot convert ‘void (n1::lla::*)(uint32_t, uint32_t*, uint8_t)’ {aka ‘void (n1::lla::*)(unsigned int, unsigned int*, unsigned char)’} to ‘cbFuncPtr’ {aka ‘void (*)(unsigned int, unsigned int*, unsigned char)’}

implementation in an online compiler

I cannot modify the software architecture because this project is a mess.