OpenWrt would be lacking 5GHz support completely, and 2.4GHz would only be partially supported due to Broadcom wifi (BCM4331, BCM4360).
Given that there has been no progress during the last 5 years, and given that this device has a Broadcom chipset (with all its implications), I wouldn't set my hopes too high to get this device supported any time soon.
Most recent topic in this forum (without support for RT-AC66U):