Following instructions are provided without any warranty, and may even get you in trouble legally. The instructions are provided for testing, learning, preventing e-waste, and should be use with care. We (including contributers + commentators) are not responsible for any damage to your device(s) or any legal issues.
Instructions have been moved to https://github.com/francoism90/asus-router. :)
Did a lot of rebooting and nvram clearing to figure out which entries broke it and tested it by going in media bridge mode and connecting it to my main router. It forces my main router in to 80mhz mode if those entries are changed and if I check the GT-AC2900 in router mode the tick box / selection for 160mhz is also gone. Nvram clear and changing everything but those three entries restores 160mhz functionality. (or one of them, haven't tried them one by one because it's pain in the ass to wipe nvram all the time)