Needed a new router today and selection wasn't great. Picked up an Asus RT-AC65.
At first I assumed it was the same thing at the RT-AC65P, but it actually has half the available RAM (128mb vs 256 on the P version, both have 128mb NAND). My next assumption was that I could just flash the AC65P firmware and it would work, but that hasn't worked so far. I've tried copying the firmware both over SSH and TFTP. After a power cycle it just reboots back to the stock firmware.
I can't find anyone that's posted anything about this particular non-P version. Is there any chance of it being supported?
Be very happy that your device didn't hard-brick (don't ever do this), RT-AC65 (ARMv7) and RT-AC65P (mips) are very different device - and the difference in RAM sizes and flash partitioning alone would (either) have been enough to damage your device permanently:
At this point there are no devices using the MT7629BA SOC supported by OpenWrt, while I'd assume this to be possible, you'd have to start from zero (mt7623 might provide some guidance, but not enough on its own).