unknown MAC address on home Wifi upon iPhone connections

I recently installed a new router and as soon as I attach my iPhone to the Wifi, I get TWO new MAC addresses. The first address corresponds to my iPhone. The second is unknown. I am trying to figure out what this unknown device would be. I have checked my iPhone's MAC address (both with "private" on and private off), I have checked the MAC address on my Apple Watch (both private and not private). None of those MAC addresses correspond to the unknown MAC address.

If I leave my house, then it shows that my iPhone's is no longer on the Wifi, but the second, unknown MAC address will remain connected.

I have tried renaming my network and making a new password. Each time I do this, the mysterious unknown MAC address doesn't show up until I've made first contact with my Wifi using my iPhone. Each time the unknown MAC address shows up (though it is different, b/c it's being randomly assigned)

Does anyone know what this unknown MAC device is? Is there an iPhone app I can use to try to triangulate its position in my house.