Thanks @Solla-topee but the problem is the device doesn't show up on Tether, nor on my router's IP table.  I can't find its MAC anywhere either, not on Tether or on the router's list.

It's literally like it's full stealth mode.  I know for a fact it's getting data because I can connect to it on my wireless with a 3rd party app, but its IP or MAC address aren't showing up anywhere.

I even tried to look through the ARP tables on my computer and it's not visible there either.

@Solla-topee the device is a wireless 900 Mhz radio that bridges between our local internet and a local 900 Mhz network.

Our overall network involved the cable modem (which acts only in bridge mode) connected to the primary router (Netgear R6700 v3).

Multiple ethernet hubs are connected on various lines from that router for a few computers in the house.

Wirelessly most other devices in the house connect to the router.

The RE450 is one of those devices connected wirelessly on a higher floor.  Multiple devices connect to it and work fine wirelessly.

The device in question is connected to the RE450 by ethernet cable (trying to maximize reliability since it's the farthest from the extender).  It used to be connected wirelessly and worked flawlessly, getting its own IP address on the network just fine.

All devices on the network (including those connected to the RE450) receive their IP addresses automatically from DHCP.  I used to have the device in question on a static IP but removed that static IP once I realized it was no longer showing up on the network list.

I cannot bypass the RE at this time, it's too far for the wifi to reliably reach to the primary router, and it's also on another floor such that I cannot connect it directly via ethernet (believe me, I much rather would do that).

edit:  Also I just reset the RE450 to factory default settings.  When I look in Tether, it's telling me it has an IP address of 192.168.1.21 in the app (which is the same as it was telling me before I did a factory default reset).  But when I look on my router's connected devices list, it shows one of our laptops on 192.168.1.21.

also also:  I've tried to assign a static IP to the device's MAC (both wifi and ethernet), and neither of them are getting an IP.  However I just connected my cellphone to the extended 2.4 Ghz network and it was instantly assigned an IP address (and the MAC showing up on the router is completely different than the phone's actual IP).