I had a similar issue at my home until I sprung for business internet. I tried switching my SMTP port but it still did not work. I had to use my ISP’s SMTP server as a smart server for postfix. Usually their SMTP server is something like: smtp.ispname.com. I guess ISP’s can pick up that you are sending emails even on another port. I never had a problem forwarding my outgoing mail through their SMTP server. It acted as if it was coming from my SMTP server.
For incoming mail, at my router, I had public port 52525 pointed to local port 25 on my postfix server. I used dyndns.com’s mail port redirector service ($50 a year) to redirect any mail to port 52525.
That $50 a year also includes spam checking and if your server is down, they will hold the mail for awhile.