I double nat because the only issue I have found is icloud relay won't work and there is only me in my house that uses that so I can work around it. The handoff and meshing doesn't work if its not in router mode. I reserved the ip address and put it in the dmz and make sure UPNP is on both routers and it seems ok. I believe I'm on the latest released firmware (not beta) but will check, don't fancy putting beta firmware on it though.
I didn't know anything about the different hubs as virgin only came to my area in December and I was sick of waiting for open reach to "switch on" fttp for me even though they have put the cable in (work was done before virgin), I would of settled for a lower speed if I would of got an older hub but from what I read I could put it in modem mode but obviously can't with the 5x.