I have a similar issue when I am visiting my parents. Despite having 30 mbps upload at my home, I cannot get anywhere near that when trying to access things from my parents house. Not just Plex either, I host a number of services. I’ve tested their wifi and download, and everything seems fine. I can also stream my Plex just fine from my friends places. I’ve chalked it up to poor (or throttled) peering between my parents ISP and my ISP. I’ve been meaning to test it through a VPN next time I go home.

I somewhat wonder if CloudFlare is issuing two different certs. An “internal” cert your servers use to serve to CloudFlare, which uses a private CA only valid for CloudFlare’s internal services. CloudFlare’s tunnel service validates against that internal CA, and then serves traffic using an actual public CA signed cert to public internet traffic.

Honestly though, I kinda think you should just go with serving everything entirely externally. Either you trust CloudFlare’s tunnels, or you don’t. If you don’t trust CloudFlare to protect your services, you shouldn’t be using it at all.

Just serve the CloudFlare certs. If the URL is the same, it won’t matter. Doesn’t matter if you’re talking to a local private address like 192.166.1.100 or a public IP. If you’re accessing it via a DNS name, that is what is validated, not the underlying IP.

PS. If you tried this and are having issues. We need more details about how things are set up, and how you are accessing them.