The SOHO gets its WAN feed 100% from cellular hotspots and has worked without issue for many years.
After multiple reboots of the router and the AT&T Nighthawk hotspot (leaving the router down for 30 minutes before plugging back in), it still had the same problem. On a whim, I swapped out the AT/&T Nighthawk for a Verizon 8800L hotspot, rebooted and everything was back to normal. That didn’t make sense as the Nighthawk has worked well for a long time and still worked well when not tethered to the SOHO.
I shut everything down again, brought the Nighthawk back up and tethered it to the SOHO. I then plugged the SOHO back in and everything was back to normal with the SOHO handing out legitimate IP addresses again to all devices.
As an aside, the client PCs only showed the bogus IP addresses when they were attached to the router. The network icon showed they were attached but it said “no internet”. Since the problem has resolved itself for now, I can’t try the ping. I should have tried that when I was having the problem, but I didn’t think to do that.
The really strange thing about this is that this happened “out of the blue” . . . no changes were made to either the hotspot, the router, or the effected clients. The hotspot and the router are on a timer that reboots them every night to keep the internet connection fresh.
Thank you kindly for taking the time to make suggestions. Troubleshooting problems with multiple device “layers” is stressful, and a fresh set of brain cells is always helpful.