NAT loopback?

[riahc3]

(No pfSense here)

My modem was replaced today and it seems that NAT loopback is not working.

Basically my modem has a IP of 172.16.0.1 I believe. DMZ is enable and it is pointing to a firewall/gateway I have which is 172.16.0.2

I turned off the FW, etc. on the modem so everything is handled by the firewall/gateway.

This configuration was basically the same before the modem was swapped and NAT loopback worked. But now, it doesn't. I don't seem to see any option that turns it on either.

Can someone point out how to do it "manually"? With some NAT rules, FW rules, port forwarding, etc.

For the record, the modem is ZyXEL AMG1202-T10B and the firewall is ZyXEL USG 50

Thank you