I experienced the same thing before, I didn't use any fancy ip thing because my ip would stay the same for a year or more, anyway when I was connected behind the router I always had to use the internal ip to access my server, when I wasn't behind it I would use the external ip or domain and it would work fine with port forwarding, I am not sure why it was like that, dmz will expose that computer to the world, so a separate firewall is recommended
Yeah I've setup my FTP server before. I had to rebuild my DC so was going about it again and this time around it was more involved than it has been in the past. I know when outside your network you need to use your external IP and forward the apropriate prts/ports the internal address of the ftp server. But i've never had issues with ports. It was strange and hard to explain but the client i use on my phone after opening a range of ports seemed to fix the issue for my phone client. However thorugh an external pc and filezilla i'd connect but fail to get a directory listing. After looking at the firewall setting within IIS 10 there is a spot to enter in your external IP. Once I did that wahla I was able to connect remotely just fine. I may remove that port range that I forwarded and see if that's even needed. I just don't recall ever needing to do that.