Setting up VPN to allow RDP to RDP

So I have been using VNC over the net to remote into my home PC, which I know is pretty stupid, so I want to create a VPN network and/or server to accomplish this securely. In short what I want to achieve is, an RDP connection from my work computer, or any public computer, securely to my home PC (what I am using now) via VPN.

This is what I have available at the moment:

1. WAN ISP connection at home, with a shitty ISP router that doesnt have VPN server functionality (Sagemcom)
2. ExpressVPN account (which doesnt appear to allow port forwarding?)
3. OpenVPN server setup on my home PC (to act as a server) with the manual config provided from ExpressVPN
4. Windows 10 Pro PC

I am at home now currently connected to the OpenVPN server with a dynamic 10.xxx.xxx.xxx address, and getting a dynamic 45.xxx.xxx.xxx address from ExpressVPN, My ISP WAN address is 42.xxx.xxx.xxx which for the most part is static (maybe changes once every 2-3 months).

I have countered all these dynamic addressing issues with DDNS that I have been using for years successfully with VNC.

I believe I may have all the tools necessary, correct me if I am wrong, but having alot of trouble in which ports to forward, where to host the VPN server (can I host the VPN server AND use it as my home machine to RDP to?)

For example, I forwarded port 3389 on the home router, ran a test over the net, and it was successfully opened. Does this just open the port? I can't see how then it would know to 3389 to my personal PC?

Any help is much appreciated.