in this case, i assume you meant to post this with the "!", correct?
so what response do you actually get when you request http://example.com/ from a non-mobile user agent?
some other issues i noticed: the RewriteRule as specified will produce a 302 status code. is this what you want or do you need a 301 here? if so you will need to specify a [R=301] flag.
i would recommend removing the index.html from the url and use "DirectoryIndex index.html" to specify the default directory index document. instead you should redirect to the root of that hostname: http://example.com/
get rid of the "RewriteBase /" - it's not doing anything useful for you.
instead of "IndexIgnore *" you would probably be better off with "Options -Indexes"