To troubleshoot this you'll need to look under the hood at the actual iptables rules, both for the NAT and the firewall.
Run the following commands to check the counters for each iptables rule, try to access the resource externally and run them again to see which counters have incremented. This will at least show you which (if any) rules are being hit.
sudo iptables -vnL sudo iptables -t nat -vnL
Post the output if you're not sure which are the relevant rules.
