Probably the case is - SMTP port 25 is used, so there's no option to change that.

If I were you, I would configure my router with Internet static IP address 192.168.1.2 (assuming you don't have anything else connected to your ISP modem except your AC1200). Then I would contact the...

@kaziel In Advanced=>Status page of you router, what's the IP address you are receiving dynamically ?

@kaziel OK. So lets try something else: open https://www.whatismyip.com in your browser and confirm your public address again. Then open your Virtual Server settings and set the port-forwarding that...

What is your connection to the ISP ? Dynamic IP ? The router time setting shouldn't have anything to do with that kind of problems, but lets see if it makes any difference.

@kaziel Can you confirm with your ISP that the 89.174.15.82 is really your public IP address and it is static (not dynamic). Configuring the Virtual Server on your router should be like that: Can you...