Hello,
we are aware of a strange issue.
iOS devices automatically pops up their mini login browser because they detect that an additional step is required to use the hotspot. But before the user can finish the process, they disconnect and connect to an other signal. Therefore the login will fail.
Is it possible that there are other free signals in your area, possibly multiple hotspot SSIDs are visible?
With apple devices, the problem can be solved if http://www.apple.com is added to the whitelist, however since http://www.apple.com has a dynamically changing IP address, once the IP changes ddwrt will still block the site. DDWRT changed to coovachilli recently which fixes the issue but i dont know if there are public builds available.
With windows phone, the problem can be similar however i dont know what the windows phones are checking.