This change in the mobile device is most probably due to a setting in the plugin Jetpack, we can’t be sure as we do not have your site’s URL. You may have another plugin whose settings may be causing this.
In case you have Jetpack, please follow the steps below to troubleshoot your problem:
Please login to your site’s admin panel and browse to Jetpack > Settings > Writing tab and scroll down to Theme Enhancements
Here you shall find an option “Enable the Jetpack Mobile theme”
Disable this option and load the site’s home page on a mobile device