try to disable WAN zone "Masquerading" on the Network > Firewall > Zones page or use mobile Bridge mode. In Bridge mode the router assigns its WAN IP address to another device. It is used instead of Network Address Translation (NAT) in order to make the router "transparent" in the communication process. Bridge mode disables all of these features and simply gives them a single specified device its WAN IP address.
You may refer to this links for more information: