One thing I noticed with the free version it appears they do not cache dynamic pages such as the forum
CDNs are generally utilized to cache resources that're requested a lot, but rarely change, such as your images, JavaScript, and CSS files; caching of dynamic content such as that produced by a forum would likely cause a huge mess on a forum as posts would be cached for the full length set for the CDN cache (you wouldn't see updates to forum posts until the previous cached version of that page expired).
I imagine one could set a custom rule to achieve such a behavior, but that kind of caching isn't really good for a forum (or any site) with ever changing content (posts).
UBB.threads supports CDN's due to the work that went into 7.6's fetch_ip.inc.php file which produces a valid user IP address when a Proxy or CDN is masking the default PHP REMOTE_ADDR server variable (since a CDN or Proxy server is a server that sits in front of your production web server). There is Linux module that can be installed on some servers to produce correct user IP addresses (mod_cloudflare), but hosts don't generally modify their global server configurations due to a single user request (which is why the fetch_ip.inc.php file was created and modified; it's a fairly complex set of checks to produce a genuine user IP address with multiple permutations for multiple services).
We can provide some basic guidance to some services, but we're not Google or the CloudFlare support desk/FAQ so we're not qualified to provide support for 3rd party services that we don't own/operate; but there are millions of resources that one can find in Google such as:
Wikipedia - CloudflareWikipedia - Cloudflare - ServicesCloudflare Support - How does CloudFlare WorkMediawiki - Cloudflare ManualKnownHost - The Ultimate Cloudflare Guide
