pfSense

When uploading custom HTML for the Captive Portal, there are options to view the current uploaded HTML in browser.

This does not work for the "Current Portal Page" when outside the internal network, because the URL of the "view" button is wrong.
The "Current Auth Error Page" view button and "Current Logout Page" view button have the correct URLs to view the site (and not open the portal itself). Opening the portal itself generates the "An error occurred. Please check the system logs for more information." error - because it is not a client that should/can authenticate outside the network (log error: "/index.php: Zone: zone_name - Captive portal could not determine client's MAC address. Disable MAC address filtering in captive portal if you do not need this functionality.")

Portal Page view: https://domain:8003/?zone=zone_name
Auth Error Page view: https://domain/services_captiveportal.php?zone=zone_name&act=viewerrhtml
Logout Page view: https://domain/services_captiveportal.php?zone=zone_name&act=viewlogouthtml

Looking at the parameters, it seems clear that "viewhtml" would work, and in fact, does:
https://domain/services_captiveportal.php?zone=zone_name&act=viewhtml works as expected.

Changing the button to this URL would make it possible to view the portal page from external hosts.