Read the first post...
I have zero problem with my RT-AC88U and the 5 GHz band. Make sure you disable Airtime Fairness, MU-MIMO and implicit beamforming. Also do a complete electrical power cycle of the router if you haven't done so.
Available channels and output levels will vary based on your region. That's why you should never buy a router from another region than yours, because in addition of being illegal to operate in your country, some of your clients might be unable to connect on the channels that are normally not available in your region.
Works for me on an RT-AC88U. What kind of WAN interface do you have? I'm non DHCP, it's possible the issue might only happen on PPPoE, or when using VLANs.
Might possibly be related to the new nvram size validation Asus has implemented in 382. Some of the nvram have very small limitations, which will prevent setting too many entries. While I might be able to adjust this on the RT-AC88U and 3100U, I can't on the RT-AC86U because part of the validation is now closed source. Same issue that made it a nightmare for me to implement support for Policy Rules on the RT-AC86U, as it would limit any new variable to 255 characters max.