Here is the few steps to solve it:
1. Go to the android to project folder
2. Then goto app -> src -> main
3. Create two directory assets then fonts into the main folder.
4. Download your custom font ,I use Roboto Font link: https://material.io/guidelines/resources/roboto-noto-fonts.html
Add this method from your activity
t1=(TextView)findViewById(R.id.text1);
Typeface roboto = Typeface.createFromAsset(getAssets(), “fonts/Roboto-Bold.ttf”);
t1.setTypeface(roboto);
That’s it
There is a video tutorials for more help.