I am have almost this exact same problem. All images are loading if I am connected to the same network as my Webwork server. If I am connected from outside the network formulas display, but other images such as graphs do not. Instead I get the error Failed to load resource: GET https://webwork.hopkinsschools.org:8443/wwtmp/AP_Calculus/images/26f1a20e-223c-3c7f-b1db-919070383517___2856e0a2-ab6e-3e60-af3c-710f82a5c79c.png::ERR_CONNECTION_TIMED_OUT. This occurs in Edge, Firefox, and Chrome.

I've tried implementing the suggestions made already but none of worked.

From sites.conf

$webwork_url = '/webwork2';

$server_root_url = 'https://webwork.hopkinsschools.org';

From localOverrides.conf

$webworkDirs{htdocs_temp} = '/var/www/html/wwtmp';

#webworkURLs{htdocs_temp} = '/wwtmp';

$webworkURLs{htdocs_temp} = 'https://webwork.hopkinsschools.org:8443/wwtmp';

$webworkDirs{equationCache} = "$webworkDirs{htdocs_temp}/equations";

$webworkURLs{equationCache} = "$webworkURLs{htdocs_temp}/equations";

$courseDirs{html_temp} = "/var/www/html/wwtmp/$courseName";

#$courseURLs{html_temp} = "/wwtmp/$courseName";

$courseURLs{html_temp} = "https://webwork.hopkinsschools.org:8443/wwtmp/$courseName";

# Location of MathJax script, used for the MathJax display mode.

#$webworkURLs{MathJax} = 'https://webwork.hopkinsschools.org:8443/webwork2_files/mathjax/es5/tex-chtml.js';

$webworkURLs{MathJax} = "$webworkURLs{htdocs}/mathjax/es5/tex-chtml.js";

This is version 2.16 installed from the virtual machine image.