Access ViewPager Fragment method from Activity

Using the ViewPager.OnPageChangeListener is the correct way to go, but you will need to refactor your adapter a bit in order to keep a reference to each Fragment contained in the FragmentPagerAdapter.

You do that using the instantiateItem() method override in the adapter, here is a simplified example:

 class PagerAdapter extends FragmentPagerAdapter {
        String tabTitles[] = new String[] { "One", "Two", "Three", "Four"};
        Context context;
        
        //This will contain your Fragment references:
        public Fragment[] fragments = new Fragment[tabTitles.length];
        
        public PagerAdapter(FragmentManager fm, Context context) {
            super(fm);
            this.context = context;
        }
        @Override
        public int getCount() {
            return tabTitles.length;
        }
        @Override
        public Fragment getItem(int position) {
            switch (position) {
            case 0:
                return new FragmentOne();
            case 1:
                return new FragmentTwo();
            case 2:
                return new FragmentThree();   
            case 3:
                return new FragmentFour();
            }
            return null;
        }

        //This populates your Fragment reference array:
        @Override
        public Object instantiateItem(ViewGroup container, int position) {
            Fragment createdFragment = (Fragment) super.instantiateItem(container, position);
            fragments[position]  = createdFragment;
            return createdFragment;
        }
        
        @Override
        public CharSequence getPageTitle(int position) {
            // Generate title based on item position
            return tabTitles[position];
        }         
 }

Then, instead of creating a new Fragment, use the one contained in the adapter:

mViewPager.addOnPageChangeListener(new ViewPager.OnPageChangeListener() {
  @Override
  public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) { }

  @Override
  public void onPageSelected(int position) {

        // do this instead, assuming your adapter reference
        // is named mAdapter:
        Fragment frag = mAdapter.fragments[position];
        if (frag != null && frag instanceof FragmentTwo) {
          ((FragmentTwo)frag).sendGetRequest();
        }
  }

  @Override
  public void onPageScrollStateChanged(int state) {  }
});

Note that if you’re using different Fragment classes in your adapter, you can implement an interface that defines sendGetRequest(), and in each of your Fragment classes implement the sendGetRequest() method.

If you don’t go with the interface approach, you will need to cast the Fragment to your own Fragment type as shown in the example above, i.e.:

if (frag instanceof FragmentTwo) {
    ((FragmentTwo)frag).sendGetRequest();
}

UPDATE

For using ViewPager2 and Kotlin, here is how it would look:

    viewPager.registerOnPageChangeCallback(
            object: ViewPager2.OnPageChangeCallback() {
                override fun onPageSelected(position: Int) {
                    super.onPageSelected(position)
                    val frag: Fragment = mAdapter.fragments[position]
                    if (frag != null && frag is FragmentTwo) {
                        (frag as FragmentTwo).sendGetRequest()
                    }
                }
            }
    )