I have the same issue. Can't configure DMZ IP address due to setting conflict with CWMP service.
Have contacted TPG Helpdesk (email@example.com) via email with no success. They have said I already have the latest firmware installed (version: 0.1.0 0.9.1 v5006.0 Build 170630 Rel.49402n).
As far as I know, my firmware hasn't been updated since I was first connected TPG NBN in August 2017.
What email address shall I send relevant details, to arrange firmware to updated to resolve this issue?