I am trying to setup almost the exact same thing.
My new ISP has given me a static block that is on a different subnet/range than the wan IP they have given me.
For example (these are not the actual IP's)
WAN IP - 126.96.36.199
WAN Subnet - 255.255.255.252
WAN Gateway - 188.8.131.52
The static block is 184.108.40.206/27
I have added the Local VLan (named DMZ) on my mx400 with the interface IP of 220.127.116.11 with vlan ID 500
I have a laptop plugged into a switch on port 23 which is an access port on vlan 500. I set the static IP of the laptop to 18.104.22.168 with a gateway of 22.214.171.124 (the interface IP on the mx 400). the laptop can reach the outside world perfectly but nothing can come into it.
my question on the 1:1 NAT is: should the WAN IP and the LAN IP of the NAT rule be the same (126.96.36.199)?
also, any 1:1 NAT rules work fine for the devices that are on my internal 10.1.0.0/16 network (even without the DMZ vlan on the mx400)