Wordpress Mobile Apps Can't Access

First Check the Log on the Apps. if there are some error message Like below

1) Unable to locate xmlrpc.php

2) Couldn't connect to the WordPress site.
There is no valid WordPress site at this address.
Check the site address (URL) you entered.

The Issue above most case by xmlrpc.php is Block by Server Permission.

For Apache on Ubuntu, edit the configuration file with the following command:

sudo nano /etc/apache2/sites-available/000-default.conf  

Find the code below in VirtualHost and Remove the xmlrpc.php Permission will Solve the Wordpress Mobiles App can't access issue

<VirtualHost>  
…    
    <files xmlrpc.php>
      order allow,deny
      deny from all
    </files>
</VirtualHost>  

*If you wish to Block Or Disable the XML-RPC From XML-RPC Attacks this is one of the Solution.

This method will Manually Blocking All XML-RPC Traffic will stop anything that utilizes XML-RPC from functioning, including Jetpack or the WordPress mobile app.