Looks like DNS-level blocking from the WiFi gateway or ISP. When a blacklisted domain is requested, it gets IP pointing to the site with that notification, not the actual IP of the requested domain.
This can be overridden by manually setting DNS servers on the phone or computer to non-filtering public DNS servers, eg. 1.1.1.1 or 8.8.8.8.
Thanks! I didn't know American ISPs would blacklist domains so easily. I'm also going to try contacting Comcast as I can't figure out what is "dangerous" about the site. It's just client-side javascript that queries content from reddit's API.
This can be overridden by manually setting DNS servers on the phone or computer to non-filtering public DNS servers, eg. 1.1.1.1 or 8.8.8.8.