RDP works fine if you have a connection to the other system. Years ago I used Hamachi to access mom's here in WI when I lived in CA. I think they went paid now.
Really all you need is a VPN connection to the machine you want to connect to if you are going across the internet.
I use RDP on my computer at home, my work laptop and my phone all with a Sophos VPN connection and have never had any issues.
If you don't have a VPN, check Zoho or OpenVPN. You are looking for a traditional VPN, not the ones that route you to other places to fool Netflix or whatever.
Howtogeek has a good primer and might have everything you need, including using Windows for your VPN. Your Router may have that capability as well.
https://www.howtogeek.com/131961/how-to-access-windows-remote-desktop-over-the-internet/