Authenticate with external Server - Mobile App

We use authentication via an external server - the latter sends HTTP status 200 or 403 depending on whether the room name is valid (200) or invalid (403). This works perfectly in desktop browsers (chrome / firefox), but not with mobile apps. There a room always opens, apparently the link here is not forwarded by nginX for checking to the external server … Thanks in advance for any information about what is different here with mobile apps.