Hopefully I explain this without confusing the living daylights out of people and it is of use to someone.
Currently we have MikroTik routers that NAT the LGW assigned address to a public as described in JNovak’s post found here: LGW router mode. This works fine for most of the customers but several require multiple IPs on different devices and refuse to accept a “192.168.xxx.xxx” address as their public facing IP. To hand off a “chunk” of public IPs, I use an L2TP tunnel to assign a single address from inside our network and then statically route the subnet of public IPs to that address. On the client side I have a MikroTik router that has the first IP of the public subnet on the customer facing interface. The outside interface is set to 192.168.254.2/24 which is set as the DMZ on the Atom R9 UE.
/interface l2tp-server server
set authentication=chap,mschap2 enabled=yes
add comment=LTEpublicFIX distance=1 dst-address=188.8.131.52/29 gateway=
add name=CUSTOMER password=PASSWORD remote-address=184.108.40.206
add add-default-route=yes connect-to=111.222.333.1 disabled=no name=l2tp-out1
add address=220.127.116.11/29 interface=ether2 network=18.104.22.168
add address=192.168.254.2/24 interface=ether1 network=192.168.254.0
add distance=1 gateway=192.168.254.1