Router assigns IP addresses from an out-of-range subnet

Problem

I have a 5031NV-030 Pace router, with wi-fi enabled. Except for TV, and AV receiver, all other devices connect over wireless. Everything is default, router is 192.168.1.254, etc. Since yesterday, all wireless devices are being assigned an address from 192.168.10.x series, which is puzzling, since the subnet is 192.168.1.x. Initially I thought some other device on the network may be running a DHCP server, but that's not the case. I did a factory reset of the router and the problem went away and re-surfaced a few hours later. Oh, and as soon as I did a factory reset, the router firmware was updated (bliking amber power light). Initially it was restricted to wireless-only, but now even wired-devices are also being assigned addresses in from the 192.168.10.x subnet. Of course, the internet connectivity is effectively down for all these devices.

I can connect to the web-UI on http://192.168.1.254/ Nothing is out of the ordinary, transmitting at 2.4G, an additional _guest SSID disabled, WPA2-PSK, 802.11b/g/n

As a workaround, I have assigned fixed IP addresses to all my devices, and they now connect fine. I am still puzzled as to where did those 192.168.10.x assignments come from.

Any clue? I can't seem to figure out the cause, and not sure what I am missing.