No Internet Access using either wired ethernet or WiFi

ibman wrote:

2013 Mac has good internet connection when connected to Deco WiFi. It also has good connection when connected to ATT RG.

When I unplug 2020 Mac from the gateway all TCP/IP settings are the same except for last set of numbers. Both are within the appropriate range of IP addresses for the connection. Both use DHCP.

Yes, and that would be the problem when multiple IP routers exist within a configuration.

Just as soon as multiple IP routers are present, routing gets far more “interesting”.

Here’s a brief intro: https://www.cloudflare.com/learning/network-layer/what-is-a-subnet/

Usual fix here is using access points or mesh (both of which are “transparent” to IP connections), or configuring each separate network segment among routers as a separate subnet.

APs and mesh can operate as one subnet.

When multiple IP routers are present, using the same subnet for distinct segments can lead to routing errors.

Routing errors manifest by an inability to connect, or by an inability to get replies. Both of those show similar results, but the triggers can be in different places in the network. What does this mean? Within network connections, IP packets do not follow the same path as the previous packet, and IP responses from the remote end can follow yet different network paths.

Either get rid of the IP routers (reconfigure or replace routers to APs or mesh), or configure IP subnets.

You stated that your 2013 Mac has good internet connection. Is that when connected to your Deco WiFi?

When you unplug your 2020 Mac from the gateway and connect to the same Deco WiFi as your 2013 Mac, how do the TCP/IP settings at  > Network > WiFi > Details > TCP/IP differ? The Router IP Address and Subnet Mask should be the same and the IP Address should only be different in the last set of numbers. Both should be using DHCP.

What is the reason for the ethernet connection to the router as well as the WiFi connection?

You Mac is only going to get an outside IP address from either the ethernet connection or the WiFi connection. Otherwise one of them will have an APIPA Address that starts with 169.254 when viewing the IP Address under the TCP/IP tab in your Network settings. This address will allow local communication, but no access outside of the router.

You can set the Service Order by going to  > System Settings > Network, then click the button with the 3 dots on the bottom of that page and click "Set Service Order". You can move which one you want to use first.

ibman wrote:

My point is that I cannot get internet whether I use a direct ethernet connection to the Uverse router or I use WiFi.

Yes. You’ve stated that. Which is exactly what can be expected when subnet routing is messed up. Packets might be able to get to the destination, for instance, but the path back to the originating host might not work. Connecting together more than one IP router or more than one NAT requires some care with the configuration.

So… is there more than one router here?

Is there more than one NAT?

If so, you will likely find yourself needing less NAT, and quite possibly more than one IP subnet.

Your IP routing hardware configuration is probably invalid.

How many IP routers here? Two? If so, each network segment needs to use its own IP subnet.

You’ll also want only one box doing NAT, and that should be the box at the edge of your network.

Unfortunately, AT&T Uverse is too often a bag of hurt with even moderately complex network configurations, too.

AT&T Uverse uses what is easily the most bizarre local network setup I’ve encountered.

Getting local servers at fixed private IP addresses was… interesting… with at least the older Uverse default network setup. The AT&T router… made assumptions… and required assumptions… about the local network. (It’s been a couple of years since I last wrestled with Uverse. Maybe it got better.)

I’d suggest checking in a forum more specific to Uverse configurations. As part of that, I’d look for a way to get Uverse box into its bridged mode (and to stop it from doing its weird stuff), add a network box performing firewall duties at the edge of your network (and NAT if applicable) connected to the Uverse bridge, and then use APs or mesh from there.

This setup doesn’t really have anything to do with the connected gear, it’s likely the generic Uverse networking mess.

If you can’t get that to work, see if you can get that Deco into an AP mode, if that Wi-Fi box supports being an AP.

AT&T may have Wi-Fi extenders available, as an alternative.

(And if you do have to run multiple IP routers for some reason, you'll need to set up subnet routing.)