Linux HowTo: How to set External IP in double nat network?

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So I have 2 routers:

1.T-Mobile B525s-23a
2.Asus RT-AC58U

My setup is

Internet -- B525s-23a ------ Asus RT-AC58U ------ devices

I want to enable DDNS on Asus RT-AC58U but it gets assigned private WAN IP so it does not work.

I dont really know how to assing my external ip to RT-AC58U. I read about DMZ on google but Tmobile router does not support that.

What can I do to get External IP on RT-AC58U to enable DDNS?

According to your information, you cannot setup DDNS via the routers, as the one
connected to the internet does not support neither DDNS nor bridge-mode.

You need therefore to set it up on the computer using third-party software.

An example free provider of DDNS is
Duck DNS:

Duck DNS is a provider of what is known as a DDNS (Dynamic DNS)
service we provide a public DNS server that anyone can get a subdomain
and use one of our provided scripts to update their record(s). so
instead of trying to remember an IP address, you can use a domain name
that is kept up-to-date by a computer at home

once this is done, periodically (usually every 5 minutes), the
computer running the client, tells our central system (via a HTTPS
post), to update the record with its latest external IP.

is another example of free provider of DDNS having a Windows client.
However, the free account will nag you every month for confirmation of your
DNS address.

You indicate you do not have an internet-routable IP address. You will not be able to receive incoming (unsolicited) traffic.

This is not surprising. Most mobile broadband services use carrier-grade NAT because there’s simply not enough IPv4 addresses available.

Dynamic DNS (DDNS) is just one thing: Fancy names for (dynamic) IP addresses. So instead of having to deal with IP addresses or even changing IP addresses, you have a fixed name to remember and pass to others. It does not solve or involve port forwarding or anything else.

If you had a public IP address, you’d want to use the DDNS client on the outermost router. Alternative, you could also use a DDNS client that auto-detects the external (public) IP address. This is a feature typically found in PC DDNS clients expected to be deployed behind NAT routers.

In your setup, you’d still need to create all port forwardings twice (though an exposed host setting, colloquially “DMZ”, could simplify things somewhat).

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