How To Configure non-standard SMTP port

If you're doing port forwarding at your router why do you need to change Postfix at all?

Tell your router to listen to port 2525 on the outside and forward the traffic to port 25 on your server.

Postfix doesn't need to know what public port it's running on. Of course, if that public port isn't 25, no other domains will be able to send you mail, but there's no need to make any changes to postfix based on what you're saying.

That's more likely to be a DNS problem than anything related to NAT.

Does the server have working DNS? Can the server resolve the client's IP address?

Assuming such is offered, look to acquire a business-class ISP service level, and move on to the next issue? The business-class services typically provide static IP address(es), opens up server ports, and sets up DNS and ISP-side server connections for your hosts. And you can call your ISP or other assistance when your network craters; non-standard connections are (as you've found) harder to support.

One subtlety here: in a number of cases, simply having a business-class service means you're supported by a different group within (larger) ISPs. This can be a significant advantage at times, given that the scripts and tools and such that are available for the business group will be rather more targeted than the scripts that are available for the residential services group at the ISP.

There are a number of folks that have sought and have tried this (try a few Google searches for SMTP and non-standard port), and it mostly works. But it is fragile, and tends to be derailed when something goes weird, or when the ISP spots and shapes or blocks the server traffic.

Also check your ISP's terms of service here, lest your network pipe be subject to summary disconnection.

The other option is to VPN or tunnel into to a co-lo or dedicated server or a virtual hosting service; to run your servers or virtual servers somewhere without firewalls blocking the standard ports.