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. :)
@khawajamechatronics
Thanks for the detailed description.
Maybe I have not yet fully understood: So, your remaining issue is that the "EU_66U: RT-AC66U B1" is still somehow in EU country mode? Despite you set it manually to "US"? And thus it is not seen/detected by your main router "UN_66U (Main) "?