1create folder in your root directory assets/Fonts and add your custom fonts
2you want.
3create a file in root folder and called react-native.config.js
4Add the following code to the file =>
5
6module.exports = {
7project: {
8    ios: {},
9    android: {},
10},
11assets: ['./assets/Fonts']
12};
13
14
15or 
16module.exports = {
17    assets: ['./assets/Fonts'],
18  };
19
20
21Then, run the following command in your terminal:
22react-native link 
23
24to use it declare this way in your styles:
25fontFamily: 'your-font-name without extension'
26
27If your font is Raleway-Bold.ttf then,
28  fontFamily: 'Raleway-Bold'

1// create a folder in your root directory /assets/fonts/ 
2// paste the custom fonts you want used in that folder
3// create a file in the root called react-native.config.js
4// Then add the following:
5
6module.exports = {
7  project: {
8    ios: {},
9    android: {},
10  },
11  assets: ['./assets/fonts'],
12};
13
14// then run 
15
16react-native link