Check your Phonegap config.xmll settings. Every URL used by your app needs to be whitelisted. Otherwise the content will not be shown.
Regards,
Ronald
[EDIT]
Example
<gap:plugin name="cordova-plugin-whitelist" source="npm" version="1.2.2" />
<allow-navigation href="https://example.com/*" />
[EDIT2]
From the documentation if you want to whitelist everthing (but they strongly recommend to not use this) it should be:
<allow-navigation href="*" />