To set up an external DB and still use your local DB, you need to edit the config.inc.php file:
On Ubuntu: sudo gedit /etc/phpmyadmin/config.inc.php
The file is roughly set up like this:
if (!empty($dbname)) {
//Your local db setup
$i++;
}
What you need to do is duplicate the "your local db setup" by copying and pasting it outside of the IF statement I've shown in the code below, and change the host to you external IP. Mine for example is:
$cfg['Servers'][$i]['host'] = '10.10.1.90:23306';
You can leave the defaults (unless you know you need to change them)
Save and refresh your PHPMYADMIN login page and a new dropdown should appear. You should be good to go.
EDIT: if you want to give the server a name to select at login page, rather than having just the IP address to select, add this to the server setup:
$cfg['Servers'][$i]['verbose'] = 'Name to show when selecting your server';
It's good if you have multiple server configs.