Your DNS records are in the 192.168.x.x range which are for private use on a local network, not over the internet. Are you hosting the site at home or over a tunnel?
Then you need to use the IP address of your router (with any required port forwarding to pass requests from the internet onto your LAN).
Your site may be working for you from your LAN, but no-one outside will be able to see it.
What it does mean is that the 500 error is due to your code or server since requests you are making go across your LAN and not the internet or through Cloudflare.