I see the same issue with Shibby 121 AIO on the RT-AC66U. On any channel, the overview page shows the 5ghz radio on channel 36 & 20MHz, but on the wireless configuration page, it shows up on whatever channel I select, at 40MHz. There is no higher channel bandwidth.
I've reflashed, cleared nvram via web page and physically, rebooted - none of the tricks seem to get it to work (they get the 5GHz radio to show up, but not work the way I expect). I'm guessing that even with the firmware set to the EU region, the hardware is physically locked at 40MHz due to it being sold in the US? Just my speculation.
I'm having other issues with this firmware - I will try going back to an earlier version before I replace the router completely.