Failed to authenticate with custom Oauth on mobile phone

Hi Community,
i have installed RC version 3.16.3 on Ubuntu server and in the setup, we use custom Oauth to authenticate user.

Now, we can login to RocketChat successfully using custom Oauth on computer ( both Windows app and web browser ).
But thing starts to fail once login on mobile app ( both Android and iOS phones ) at step connecting to Oauth url.

  • On Android, error log: ERR_CLEATEXT_NOT_PERMITTED
  • On iOS, error log: The resource could not be loaded because the App Transport Security policy requries the use of a secure connection

The issue as i understand that RC app on mobile is sending http to Oauth server but http is not allowed, instead it requires https.
Now i have been stuck here for awhile due to not finding the settings that allow Rocket Chat to send only https to Oauth server.
So anyone can help me out ?

The server is Ubuntu 18.04.2 LTS
Both Rocket Chat and Oauth servers are using reverse proxy Nginx and Letsencryted certifacates.
Thanks alot.