Port not opening despite forwarded port, no firewall and an open listen server

Question

On my previous PC, I was perfectly able to open ports with no issues, however after a switch to a new PC, all ports refuse to open.

To illustrate the process I have used, I have done the same with the test port 1235.

1. Forwarded port on router.
2. Created a listen server using this tool
3. Checked that the port is listening using netstat -an, where it reports a port listening at 0.0.0.0:1235.
4. Checked that the port is listening locally using PortQryUI , where it reports that TCP port 1235 is listening.
5. Checked that the port is listening externally using PortQryUI, where it reports that it is not listening.

To make sure that firewall is not the issue here, my firewall has been turned off and all third party firewalls have also been turned off.

I have searched around for a while, but I can't for the life of me find a solution that works for me. Any help would be greatly appreciated.

Edit: This happens for all ports, 1235 is just used to show the process.