Unfortunately, Facebook does not display non-default tabs in the mobile version of your Page. This is a universal problem and not something unique to POWR. There is currently no way to enable a POWR tab on the Facebook mobile site.
If your POWR Plugin is an essential part of your business' Facebook Page, you might want to add the direct link to the Plugin to your About tab. To get the direct and public link to your Plugin, you'll want to make use of the Share option inside your POWR Plugin. Read more about how to do this here.