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Web browsers are very sensitive to .htm and .html But on a mobile device as long as you do not specify index.htm or index.html in the page address (URL) the browser should look for and automatically load the index page.

If you have published your site with the default .htm extensions and you have existing files on your computer that use .html then the browser will load the .html pages, even if they are older.

This does not sound like what you are experiencing.

Can you give us a link to your site so we can have a look and see if we can determine what is gone awry?