As I said in my last message, if there were more news I would post an update. The configuration I described in August worked, but not completely to my satisfaction. With the Pi-hole connected to the AT&T modem/router the Deco had no problems, but the Pi-hole did not see individual LAN devices, everything showed in it's logs as the Deco router. (The Deco was in 192.168.1.x and the AT&T was 192.168.0.x. The Deco actually being 192.168.1.1 and the ATT 192.168.0.254. The Pi-hole was at a reserved 192.168.0.253).

I changed the setup so that the Pi-hole took over the DHCP server role in addition to doing the DNS. (Of course, the Deco has to have DHCP disabled, too. The only way to do that was to change the Deco to Access Point mode.) I put the raspberry pi on a fixed IP address, and it puts that into every DHCP configuration that it gives to clients. The network has been running without any issues for a few months now. I also set some other devices on my LAN to reserved addresses, but that was for my convenience for other needs. The Pi-hole logs list each device with its own ip and description so that issue has been solved.

The current configuration puts the ATT modem at 192.168.0.254, the Deco is (reserved) at 192.168.0.1 and 192.168.0.3, the Pi-hole at 192.168.0.2. The Deco is in AP mode, the ATT router has wi-fi disabled and DHCP disabled.