As other probably said. Don’t do it. Payment processing is not just setting up a server and a database. Just as implementing your own encryption protocol is a bad idea, creating your own paiement system sadly also is…

Not a cloudflare expert but have you tried specifying your public IPv4 ? You still access cloudflare servers through the internet even when in your own lan, so it’s probably your public IP address that you should add as a rule and not your private one.