Yeah just like in @calisro case it works fine. I first had it work with Lets Encrypt and then switched to origin certificate since it lasts for 30 years and apparently better. You have to turn origin checking on under SSL in Cloudflare, then generate it and install it on NPM under SSL tab. Then enable strict checking in Cloudflare. All that works with local domains being HTTP and HTTPS as I have both.
You can also use CloudFlared HA addon which basically automates everything for you and you don’t have to forward any ports on the router… although it doesn’t support sub-locations for domains to my knowledge like example.com/plex . Subdomain like plex.example.com is fine though.