So lets start with the story. I use a D-Link DWR-921 C1 modem, (it isn't an optus modem, I know so if you mention that I will ignore you) I have portforwarded all ports associated with Battlefield V and Xbox Live e.g. 3074 etc about 6-7 in total. Anyway I have spent over 2 hours talking to a guy at D-Link confirming I have done all correctly and done the remote access thing so they could check over my settings which they said is all right and they think the problem is on Optuses end.
Now the question: Has anyone resolved this moderate NAT type issue and UpNP not being successful on Xbox One with this modem type or mobile broadband before? and, If so, how?
(Answers I will ignore or treat with contempt depending on my mood include but are not limited to: Did you turn it off and turn it back on, did you follow Microsofts UpNP troubleshooting guide, is UpNP turned on, are you sure you got all the ports, and finally have you contacted D-Link, Optus or Microsoft)
My Mobile Broadband (and mobile handset) providers use whats called CGNAT, where thousands of users share a single public IP address. As the IPv4 system has a limited amount of public IP addresses (and a block of them now is around $300k to buy) providers use CGNAT to cope.
Under CGNAT you dont have any form of port forwarding as your all sharing the same IP, though this does not stop you from using the internet.
There is no way around this, public IP addresses for mobile services are not available though Optus on 4g (unless your an enterprise cusotmer).
Though I didnt have and isues with battlefield 1, nor Fallout 76 with CGNAT - works fine for me.