For anyone who is as smart (read: really dumb) as me and enabled Webmin's IP Access Control feature but forgot to add their server's IP address as an allowed IP address, here is how to regain access to Webmin after your local IP address changes and Webmin locks you out.

Edit /etc/webmin/miniserv.conf and change the "allow" IP address to your new IP address.

Now is also the perfect time to add your server's IP address to the whitelist, so that you can at least access Webmin locally from the server via lynx if you need to in the future.

To add multiple IP addresses, just put a space in between them.

Example:

allow=12.34.56.78 98.76.54.32