How to add a subdomain on a different droplet/server?

I have 2 servers on which one have wordpress site on root domain and i want to open a discourse forum on other server on my subdomain And my root domain is using cloudlflare.

So when i tried to add a dns of forum and pointed it to the new server, it still shows nothing. I dont know why , i know this is somewhat off topic. But please i am trying to figure this out from couple of weeks.

When i visit the, it shows me the cloudflare page but says the host in not working. I also added the subdomain in my digitalocean panel still nothing

See Running other websites on the same machine as Discourse. There are a few others if you search.