Virgin Media - Modem Mode Doesn't Work

I have been having major problems with modem mode. It was initially working with my SH 3 (purple light) connected to an Apple Airport Extreme. The Airport is set to DHCP & NAT mode. Last week I woke to the SH3 not working, called Virgin who reset the modem back to non modem mode. But when I re-enabled modem mode it just didn't work so I had to revert back to using the SH3 in router mode.

I have since been reading online that the SH3 has problem working in modem mode and it was recommended I get a SH2 AC which I now have. But when I enable that in modem mode my Airport has a status of "Double NAT". This means that the SH2 AC is still handling the routing etc?

This is really annoying as I am not getting the use of my Apple Airport Extreme and Express and my home does not have full wifi coverage. I thought I would post on here first before I call Virgin yet again. I have tried everything and spent out of my own pocket and still not working, not happy!

I have a post about it on the official forums but thought I would get advice here as well:
http://community.virginmedia.com/t5/QuickStart-set-up-and/Modem-Mode-Doesn-t-Work/m-p/3238844#M65680

Do you have another router to try?

Just the SH3 and the SH2 AC

I mean apart from your Airport Extreme to try the VM hub in modem mode.

After a lot of reading this is down to an known issue with the 9.1.88T firmware, its known as port fapping and is a bug until the latest firmware is pushed to the SH3

Can you leave the VM hub in router mode and use the Airport as an access point? You'd then at least have its wireless available.

The known SH3 problem in modem mode is very specific to an issue with regular 34 minute disconnects/reconnects when it is connected to an Arris/Cadent CMTS. If you are having a different problem then there is something else wrong.