Yes I access V6 using 6to4 tunel. The ZyXel doesn't support this function. I had a VMG3925 but 5ghz was very unstable on latest firmware. I googled this problem and came across a ZyXel forum thread with this problem and the fix was to downgrade the firmware to one of the very early firmwares, which I tried and subsequently bricked the 3925 completely and I can no longer log on to the web interface after many factory resets.
I think this is the way to go, using my purchased 8924 as a WAP until such times as the Asus RT-AC58U reduces in price.