issue: facebook/react-native#4415
https://ste.vn/2015/06/10/configuring-app-transport-security-ios-9-osx-10-11/
https://stackoverflow.com/questions/43277059/fetch-doesnt-work-in-react-native-ios
In case someone else needs it. I solved it by adding
<key>NSAllowsArbitraryLoads</key>
<true/>
to a Info.plist file which is located inside ios folder in your project.
Thus I changed
<key>NSAppTransportSecurity</key>
<dict>
<key>NSExceptionDomains</key>
<dict>
<key>localhost</key>
<dict>
<key>NSExceptionAllowsInsecureHTTPLoads</key>
<true/>
</dict>
</dict>
</dict>
with
<key>NSAppTransportSecurity</key>
<dict>
<key>NSAllowsArbitraryLoads</key>
<true/>
<key>NSExceptionDomains</key>
<dict>
<key>localhost</key>
<dict>
<key>NSExceptionAllowsInsecureHTTPLoads</key>
<true/>
</dict>
</dict>
</dict>