Issues getting IPV6 working via Optus FTTH NBN Australia (ISP) on a Nighthawk X6 R8000

Hi all.

I have a Nighthawk X6 R8000. 

My ISP is Optus (Australia) via FTTH NBN.
I recently had issues where I could be online, however was unable to play Helldivers II with my wife who was in the next room as an anniversary activity.

I could connect to the game servers, have other people join me, and other people join her, however, I was unable to join any games with her, and vice versa.

I have done the following:
1.  restarting PC's,
2. restarting games from desktop
3. verifying game caches in steam
4. checking for Windows Updates on both PC's that had snuck in and temporarily disabled things until a restart
5. confirming up to date networking and display drivers etc

I found a steam forum post that blamed NATs as a potential cause. I checked my NighthawkNighthawk X6 R8000 router and I have no NATs active that could affect this. Only one for PLEX that otherwise does not effect online activity. I disabled it to be sure and there was no change to the Helldivers experience for my wife and I.

Other posters indicated that it was an IP conflict. I confirmed no internal conflict on IP addresses (DHCP is set internally, only one router is set as a DHCP server on the LAN (I do have a second router on the LAN, an Optus provided 'Sagemcom' router, however I have confirmed it has DHCP disabled, and it is only used for VOIP home phone service) - only due to the fact the NX6 R8000 does not support this feature. Regardless, both can access internet and each other on the LAN (shared files and folders) without issue.

Another post indicated that using a VPN solved the issue, and after turning mine on, I was instantly able to join my wife's Helldivers squad.

While awesome, this had me considering root causes. I have since reach out to my ISP to see if they support IPv6, and after originally being told no, and persisting, they now say they can, however I now have the issue of configuring my router to make that work.

I have consulted the forums for Netgear, and discussed at length with the ISP, however, am not able to get IPv6 working for my external (internet) connection. I have tried all the 9 options available on the Nighhawk browser portal. The 'auto detect' and 'auto config' options do not resolve in a connection.

Ideally, I'd like to keep IPv4 for the internal LAN, but if not an option then so be it. '6to4 tunnel' would be nice, but I gather that is v4 external and v6 internal?

Any assistance with what settings to use on my router would be appreciated, or advice on how to get to the ISP to support me in connecting via IPv6 would be great too. They recon it should work 'by enabling IPv6 on my router' but its not that simple it seems.

TIA