Well lets say even if i have DynDNS (or other public DNS service)…i would need to set this on my router (not every router has this option) mine has, but i dont have public IP from provider (im on his local LAN) so i cant set DynDNS
I have just few ports open from ISP
so at home i use IP 192.168.1.200:5555
outside https://port.isp.com:5555 if i use this address at home it wont work.
Even when i tried to open port and use it for DynDNS it didnt worked out.
For example on android torrent client “Transmission” you set account wit some IP and then you can set also Alternate IP, always one of those works and i dont have to worry about other settings.
hope is more clear now 
Thanks