I've done everything. I can't remote desktop control my PC From outside my local network (external internet) via Windows Remote Desktop.
After lots of troubleshooting:
I can log in to my RDP host via the local network with the hostname or local IPV4 Address. So the server details are correct (account name, hostname, password).
When searching for what's my IP Address on Google (I get this IP: 187.216.212.201) It's different from what shows on my modem router getaway page (I get this IP: 139.54.239.16). And both are different from my local IP Address (I get this IP: 172.169.215.73) -definitely as they are external vs internal-.
I went to my Windows firewall and allowed the remote desktop port 3389. And I used port 3389 everywhere (router, pc, firewall...), so I don't mess up any port forwarding setting.
So I have concluded The problem stems from the external IP Address... But at which part, this is where I need your help, so I can try to fix it.
When trying to connect to a host locally, I enter the local hostname and then enter.local. It works just fine. When being outside the network (using external internet). I connect via an external IP address instead of the hostname followed by ":port#"(like 187.216.212.201:3389), but my iPhone(client) can't connect to the host, I receive the error code 0x204.
I tried all the external IP addresses I could get my hand on (the Google one, the modem router one) on my iPhone to connect to my PC windows.
I tried this service to check if my port is open, It is not. It keeps saying port is closed: https://www.yougetsignal.com/tools/open-ports/
I set up my IPV4 Address(local) to static IP via router settings + windows settings. *BTW, I use a Deco Modem Router which is very new and advanced (has lots of features). My host runs windows 10 lastest build.
I set up port forwarding on my modem router to go to my IP Address of my host. that has the same internal as external port(3389).
** Host is the desktop I want to connect to. The client is the iPhone I use to connect to the other desktop.
** The IP Addresses are random. soley for the purpose of the question.