Your router dynamically assigns your IP address within the subnet. That IP is subject to change at any time, whenver your lease expires or certain other events occur.
Most routers, probably including yours, will forward a port only to a particular IP address and no other. Hopefully, it’s to your computer’s IP. If the computer address changes (and there will be no notice to you that it has), then the listen port is not open on the router firewall for that new IP address. If some other machine on your subnet gets your old IP address, then it has an open port it doesn’t know about. Bad for security, that.
You will have to figure out that your IP has changed, find out what the new one is, and change the router firewall rule to point to the new IP. You’ll have to do that all over again, each time your IP changes. You’ll have to stay on top of it, constantly check to see whether your IP has changed.
Far, far easier to use a static, unchanging IP for your computer, and forward to that address. It only has to be done once, then you can forget about it.
Firewalls are not unlike a series of gates or doors. Only one has to be closed to block traffic, and it doesn’t matter that the others are open – the traffic is still blocked by the one that’s closed. When you change your listen port, all of the firewalls must be changed to close the old port and open the new one. This means your router’s firewall and your computer’s software firewall(s). The change must be made on the firewalls and in the BitComet preferences as well.
Some routers working with some software use UPnP to configure firewalls automatically. It doesn’t work nearly reliably enough, but you may be one of the fortunate few. It’s usually not worth bothering with if you don’t know exactly what you’re doing, and often not even then.
Generally speaking, you should have only one firewall. If you have more than one, they WILL stumble over each other like this.