Disable the legacy web client for mobile devices

The new, responsive web client works much better for mobile devices than the old one. However, there doesn't seem to be a way to disable the legacy mobile theme. When visiting the Serv-U MFT login page on an Android device, you still get the legacy themed login page. Once you login, you still get the legacy pages which are quite horrible.

If you try to login to the Serv-U server, from an Android device, via a URL which contains WebClientNew (e.g., h**p://, you get a 404 page not found.

How can you force the new web client for mobile devices?