Themekey with mobile detect not working properly
The mobile detect module had some issue with caching. The Drupal 7 Core caching have been supported since few weeks ago by a sub-module call "Mobile Detect Caching."

However, the developer mention that he boost probably would not work.... Here is what the developer said:


  It will only work with Apache(?). I doubt it will work with Varnish,
  Boost, or similar. I suspect it will work with {cache_page} in
  alternate caches, but I didn't test this.


In the comments you mention that you are using a Shared Hosting and you depend on the boost module and what you should do ? 

Well, I would recommend you to follow the issue Drupal Cache + Mobile Detect Conflict where the still discussing this module caching issue. In addition to follow you should ask for boost support or an alternative. They might try to implement this module with the boost module.

In the mean time, I would recommend to used a free CDN called CloudFlare and let them generated the caching instead of the boost Module. However, you probably would not get the same results as the Boost module.