Hi nrtrush,
I think I went a little overboard on my research, but my Xbox One was set to NAT Strict, and I had a feeling that it was going to be tough to achieve NAT Open coming from a Strict setting. So after quite a bit of reading up on this, I ended up with a NAT Open setting by doing three things (and the first step may not even be needed).
1) I added a static DHCP lease for my Xbox One from the LAN Settings page (by going to the Almond+ admin page).
2) I went into OpenWRT, then Services (from the top menu), then UPnP. My UPnP setting was enabled. I had to disable it, then hit Save, then enable it again, and hit Save.
3) I then turned my Xbox One off, and then back on. Once I did this, my Xbox was showing NAT Open.
I hope this helps, it seems like a simple fix but I'll have to see if the NAT status remains in an Open state.