Problem with mobile_theme theme switcher module
mobile_theme wouldn't work with Drupal cache on. It is not mentioned anywhere in the documentation. We should use m.example.com for mobile url's for Drupal. I tried many such modules except for ThemeKey but the same problem( since I was using the cache). 

I finally wrote my own module. But later found : simple_mobile_redirect to be good one but have not tried yet.