Carrier Grade-NAT

Internet providers can assign an exclusive IP address to each client, but this is expensive, and as a result most home users are behind a Carrier Grade Network Address Translator (CG-NAT),

What it does: Multiple users are assigned the same IP address, and the NAT interface is responsible for figuring out where to direct each client's connections.

How it works: The CG-NAT keeps a translation table that maps all IP addresses, ports, protocols, and orchestrates all incoming/outgoing packets to their respective destinations. Any incoming connection that does not exist in that table is dropped before it can reach the client.

Here's a metaphor: Imagine you live in a floating island, and the only way for anyone to reach your house is through a tunnel that only you can open. When you open that tunnel, you can only open it for one specific person, but you can have multiple tunnels. When you close that tunnel, even though the person knows where you live and how to get there, if they try to create their own tunnel, when they get to the end there will be no door.

In shorter terms, any incoming packet will be dropped unless your machine have initiated the connection. Because of this, other peers cannot connect to your torrent client directly, instead they have to wait for your client to try connecting to them, which can delay the connection or prevent it altogether if the two of you are behind a CG-NAT.

How to check if you're behind a CG-NAT:
Open your terminal and run this command:
tracert -d 8.8.8.8 if you're on Windows or traceroute -n 8.8.8.8 if you're on Linux.

You'll see a list of all IP addresses that are part of the route to Google's DNS server.
You must look for any address falling in this range: 100.64.0.0 to 100.127.255.255.

If you see an IP that fits that range, you are behind a CG-NAT, and Port Forwarding is ineffective.

For example, this is my result:

1    <1 ms    <1 ms    <1 ms  192.168.18.1
2     2 ms     2 ms     2 ms  10.255.255.9
3     2 ms     2 ms     2 ms  100.127.249.217
4     4 ms     3 ms     4 ms  200.153.91.173
5     *        *        *     Request timed out. 
6     4 ms     4 ms     4 ms  72.14.220.222
7     5 ms     4 ms     4 ms  142.251.238.17
8     5 ms     4 ms     4 ms  74.125.37.135
9     4 ms     4 ms     4 ms  8.8.8.8
Trace complete.

As you can see on hop 3 the IP address 100.127.249.217 is inside the CG-NAT range. Therefore, I'm behind a CG-NAT and port-forwarding is not possible.

What to do in this case:
You can use a VPN with port-forwarding support, like ProtonVPN or PIA (there are a few others).
A regular VPN will not suffice, incoming connections will be dropped. It has to be a VPN provider that supports port-forwarding, they will assign a port exclusive to you that can be used in your torrent client to make it reachable to other peers.

No CG-NAT and still blocked:
Sometimes, even when you have an exclusive IP, the ISP may block all ports, or ports from a specific range. In this case, if you try multiple port numbers and they all fail, you need to contact your ISP and request them to open the specific port number that you want to use.