Like @WadeAlsup said, you need use 1:1 NAT feature on the MX. First create a VLAN that will be used on the MX connecting to the Cisco 800. Use a subnet you will use only for that device. Set MX IP address and the 800 to have the WAN port on the same subnet, with next hop to be the MX. On the MX's port to the 800, change it to be access port with the VLAN you want. On the 1:1 NAT, use the public WAN IP address that you need for the 800 router and point to the internal LAN IP address of the 800's WAN port. You should now have access from the outside to inside through the MX. Depending on the client's network needs, if they need access to your network, it may require static routing set up on the MX.
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