Hi Keith,
it would be useful if you could provide a PCAP file of the Gb interface and the GTP traffic for this, so we can look at the actual ip address allocation as it happens in the network.
Please note that the PPP you speak is only between the modem and the ppp daemon, and there is no PPP over the radio interface. The IP address allocation happens via the PDP context activation on the signaling plane.
Regards, Harald