Packet tracer ICMP problem across Cloud-PT - simulating an ISP

OK so I'm trying to simulate the access part of an ISP. Its proving rather difficult though.  I can get all the clients to ping back to the interface on the ISP_EXG_R1 OK. I can dish out IP's to the customers WAN interface of their routers.

I can't for the love of me get the 'customers' public IP's to ping each other. Tracing a packet sees it being dropped on the routers interface where it should send the packet back out.

I have tried a 'ip route' but that did not work. I'm wondering if there is a limitation here. I can mapping multiple modems to one Ethernet cable...