DDNS is configured correctly and the hostname works locally, but I can’t access it from outside my network. Remote management is enabled and set to port 80. Any idea what I might be missing?

You're a little thin on details about "my network" but for the purposes of this discussion I'm going to assume you are talking about a typical home network comprising of a Router and a bunch of devices connected to it.

Let's think about the network route required to get from one computer on your network to another, and this is what I expect is happening when you say "works locally". The router knows about every device on its network and knows how to route traffic to each.

Computer A --> Router --> Computer B

Now let's consider what I propose could be happening when you make the same request using your DDNS service. There are two scenarios that come to mind:

Computer A --> Router --> ISP --> DDNS Service --> ??
Computer A --> Router --> ISP --> DDNS Service --> ISP --> Router --> ??

In both scenarios your request leaves your network via your Router and your ISP and arrives at the DDNS Service.

In the first scenario, you have the wrong IP configured with your DDNS Service so the request goes to the wrong place from there.

In the second scenario, the request makes it back to your Router but the router doesn't know where to forward it on to. In this case you will need to configure Port Forwarding on your router. What Port Forwarding does is tell the router "when you receive a request for port x, please forward it to this IP and port on the local network". The ports don't have to be the same, so you could configure requests for port 8090 to forward to 192.168.0.25:80, for example.

Check the documentation for your Router model to learn how to configure port forwarding.

My experience is that AT&T fiber for residential use does allow servers and does not block ports. I used to have Verizon FiOS which, also, did not block ports, but that was a business account and so under different rules. Spectrum and Optimum Online business accounts also didn’t block, but I paid an extra $10/month for a static IP back then as well.

It might be the router that is blocking. Most routers these days block port 80 by default. This might be the router built into your cable modem as well.

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