Does anyone have a digital system that is as involving as their analogue front end?

Here’s what Gemini says about the issue…

1. The Multi-Band Separation Fix (Most Common)

By default, AT&T combines the 2.4\text{ GHz} and 5\text{ GHz} Wi-Fi bands into a single network name (SSID). Many dedicated music streamers—especially older models—only have 2.4\text{ GHz} Wi-Fi chips. If your phone connects to the 5\text{ GHz} band and your streamer is on the 2.4\text{ GHz} band, the AT&T router sometimes blocks them from talking to each other.

• The Fix: Type 192.168.1.254 into a web browser to log into your AT&T Gateway settings (the access code is printed on a sticker on the router).

• Navigate to Wi-Fi > Advanced Options.

• Give the 2.4\text{ GHz} and 5\text{ GHz} bands different names (e.g., "HomeNetwork" and "HomeNetwork_5G").

• Connect your phone and your music streamer to the exact same network name (ideally 2.4\text{ GHz} for the streamer to ensure stability).

2. The IP Address & Subnet Mismatch (If Using Ethernet)

If your streamer is plugged directly into a wall jack or a network switch, it might still be clinging to an IP address given to it by your old internet provider. If your old network used a different IP scheme (like 10.0.0.x) and AT&T uses 192.168.1.x, the streamer is essentially speaking a different language.

• The Fix: Unplug the power cable from the back of your music streamer, wait 30 seconds, and plug it back in. This forces it to request a brand-new IP address from the AT&T gateway.

• If it still isn't recognized, you may need to perform a factory reset on the streamer itself so it forgets its old network lease.

3. The Double NAT Conflict (If Using Your Own Router)

Did you plug your old Wi-Fi router or mesh system (like Eero, Google Nest, or Netgear) into the new AT&T box? If so, you now have two routers trying to direct traffic. Your streamer might be connected to the AT&T Wi-Fi, while your phone is connected to your old router's Wi-Fi. Because they are on completely separate networks, they cannot see each other.

• The Fix: You need to choose one system to handle your Wi-Fi.

• If you want to keep using your own personal router, log into the AT&T Gateway (192.168.1.254), go to Firewall > IP Passthrough, and turn it on for your personal router. Then, turn off the Wi-Fi interface on the AT&T box entirely. This forces AT&T to act purely as a modem, leaving your personal router in charge of all your devices.

@audphile1 

Thank you for the information. I have turned off the second subsidiary mesh that the streamer was plugged into, so there is only one mesh plugged into the fiber hub. I now have the Meitner streamer plugged directly into the fiber hub which has four ethernet plugs. After plugging the Meitner directly into the ethernet hub, it did not connect to the iPad, which is what I use for Jplay. I turned off the Meitner and unplugged it for several minutes and also unplugged the fiber hub for several minutes. I rebooted the fiber hub. Then plugged in the Meitner and rebooted it. Still no connection. I do not think I have enough information about the Meitner and how to get into its network info to do what you're suggesting. I sent in a request for support to TMR. They should know all of this network stuff and be able to help. Or perhaps they can connect me to Meitner. I've spent the last several hours trying to do it myself and I'm burned out. But I have your information to tell to TMR support if they need it. Thank you. 

Give EMM Labs a call if TMR is unable to help

Can you try mconnect app? It’s free. See if that recognizes the Meitner 

One other thing to try is to go into the at&t router or mesh setup and see if upnp is enabled. If it’s not, enable it. This can potentially fix your problem. 
I googled this and at&t routers moght not support upnp. If that’s really the case then you need to use a 3rd party router like netgear os something similar and plug thd Meitner into it. 
It’s unlikely the Meitner that’s a fault. 

What happened to the other mesh network you were using before when it all worked? Can you switch back to that?

I'll try switching back, but it didn't work on the other mesh either. As I said, I had the same problem with my Moon 280D after a black out. Moon had to help me get it going again. Thanks. I hate network problems.