Geez, has it really been 2 weeks?!? Sorry guys for not checking in.
I could not make any progress with my experiments. I built the smallest simplest network, eliminating as much as possible and couldn't get it to work. I removed the eero completely and moved everything back to the ATT all in one router. But only after I did a factory reset of the ATT router did it all start working again.
The NAS computer still doesn't appear as a computer on the network in the music server PC, but I can ping it and browse the shares so I can stream music from the NAS thru the server to the client again and it's working good enough. That was the main problem in my original post and it is fixed. I don't know what was wrong with the router setup that caused one computer to not connect to another. But there were several problems like this, it almost seemed to be getting worse as the connectivity problems seemed to be increasing.
According to the ATT tech I called on Saturday, she dialed into my router and saw that I had disabled the wifi on the router. I did this to allow the eero wifi network to take over the LAN duties while minimizing the influence the ATT router would have. She said that disabling the Wifi on the ATT router makes them go bonkers. It is very sensitive to this. The 2.4GHz and 5GHz Wifis need to both be active, and have same SSID, and even have the same password "for best performance" it says right in the setup GUI. Disabling the Wifi apparently was one of the main problems. But she still couldn't get it all straightened out, so I asked if I could just factory reset to restore basic function. So we did that and it came right up. I just left it in that basic default configuration, just changed SSIDs and password, etc.
She also said that this ATT router is also very sensitive to other brand WiFi extension networks, like eero, etc. She said only Netgear (and another brand I don't remember) are compatible with this router as extension WiFi. I assume that means when you leave the router's WiFi enabled, which now I know is required. So if I need stronger WiFi I will get a Netgear extender. I will ditch the eero system.
I truly appreciate everyone's interest and help!
Thank you
Rich