Currently the https://developer.webex.com/docs/api/v1/meetings/join-a-meeting API does not work with mobile devices. You can enter the join link in a mobile browser, but it will still try to redirect you to the Webex app. The same join meeting link that this API generates will work in a Desktop PC's browser.
The request is to add support for direct join on mobile devices using the link the API generates.
The old XML API and events was capable of this, the new REST API with Webinars does not support this, so this is a feature gap.