Setting up your network allow Remote Desktop connection can seem like a very daunting task, however it is not as complicated as it might seem. Keep in mind through this process you are opening pathways into your network from outside that anyone can access, make sure your passwords are complex and your not using a common username(ie. administrator), and all will be well.
There are essentially two tasks that need to be completed in order for you to Remote Desktop from another network.
1. Setup Dynamic DNS. Quite simply Dynamic DNS is a service that keeps track of your networks ip address, and allows you to give it an easy to remember domain name. This is not necessary, but I would MUCH rather remember MyHomeNetwork.dyndns.org, than 18.104.22.168(and this number changes on a weekly basis), no thank you, Dynamic DNS is much more convenient.
2. Open ports on your router to allow traffic through. I think of network ports as pipes in and out of a building. Windows Remote Desktop and VNC use different ports, just like water and gas use different pipes going in and out of a building.