I have never had anything less than Open NAT on any Xbox console with various ASUS routers over the years, but my PS4 with the same network hardware was always NAT Type 2 and nothing would change it to Type 1. However, this never caused me any issues in any multiplayer or online co-op games though, so I never put any effort into chasing Type 1.
SH2 modem mode | AC86U router | AC68U node