Geolocation - establish a secure connection issue

Hi all,

I'm helping one of my students try to develop a web app for his ecology class that depends upon geolocation. It's necessary that it works on mobile devices, however, we've now tried many different cell phones and many different browsers and we keep getting "cannot open the page because it could not establish a secure connection to the server." We've run lots of C2 apps before with no issues on this server.

Any insights? Here is the simple test page. It should work just fine on a PC but no luck on cell phones.

https://srealist.neocities.org/GPS/index.html

I greatly appreciate your insights!