Remote Access - Very Strange Certificate Error

My father was having issues with duckdns, so I set him up with cloudflare and the NGINX PM addon. Same setup I’m using for myself.

At his home, we can access HA through the IP or through the FQDN without any issues.

When connected to mobile data, we can access HA through the FQDN without any issues.

But as soon as we connect to a WiFi network that is NOT his home network, we get an SSL certificate error of “ERR_SSL_UNRECOGNIZED_NAME_ALERT”

I’m stumped because this doesn’t make any sort of logical sense to me. Cleared cache on all devices, cleared cache on Cloudflare. Even went to the local coffee shop to test a different WiFi network and get the same error on their network. Happens on laptop, tablet, and mobile phone browser as well as app. Tried it on Chrome, Edge and Firefox as well as incognito modes of all 3. Even went so far as to whitelist his domain on my side in case adguard was blocking it.

I can’t see how mobile data can connect with no issue but any WiFi network other than his home network can not. Anybody have any thoughts here? Could it be something ISP related?