Nighthawk MR1100 offloading routing problems !! Annoying !!!

When the Internet is running via the 4G gateway on this Nighthawk MR1100 all devices can see each other with their respective IP addresses assigned by the Nighthawk.

BUT when I use Data offloading - which in effect is using a different gateway by connecting to my wireless router instead of the built in 4G - these same devices now CANNOT see each other on their respective IP addresses assigned by the Nighthawk.

Where is the logic in this? It's driving me crazy. I've just changed the gateway out?

I need a solution so that when if I offload data to the Wi-Fi the devices connected to the Nighthawk can still see each other.

Please help!