I'll share w/ you how to enable this page
Keep in mind that's a temporary page, the Oxwall team is working to bring the join page to mobile version. It's not perfect but works until there is a mobile version
You'll need:
1- The mod attached in this post (on the comment #9)
2- Cache cleaner extreme plugin or Oxwall Profiler enabled in your ow_includes/config.php
Just download the attach from this post & upload to your website
Extract the file in your website root
Clean the cache using Cache Cleaner Extreme or Oxwall Profiler mode & the join button will be available on the mobile version
P.S.: Need some improvements