webeindustry It's like the OVH hosting, there are many people for who it does not work and many people for who it works... (I'm in neither case, I don't use it but saw it about everywhere on the forum)
Cloudflare works fine for me with Flarum & Nginx. But I don't think I tried to convert IPs. I do the IP settings in the Cloudflare Firewall and try not to track my users too much.
thunder Where are you stuck ?
You need to install/enable that module (it's not from Cloudflare, it's a standard NGINX module). I'm not sure how to do it and it depends on your system/config.
Then in your site config you need to add the items they show you. This is the list of Cloudflare IPs (that, as they say, will need to be updated from time to time). When the module sees one of these IPs, it will replace it with either the
X-Forwarded-For header (that's what you choose on the last line).