If traffic from your mobile clients have a common parameter you can filter them on, then you may want to try and set up your own firewall rules.
The Under Attack Mode basically just serves up an interstitial page to all visitors. With Firewall Rules, you can define your own parameters on when to serve the challenge.
Example of such a common parameter could be a (partial) user agent string. For example, I know that many social media apps include their brand name in the user agent when a page is opened through their app.