Hi t4alha,
Every device is different. You will have to check the documentation for the device that your ISP gave you. For example on my backup internet connection I am using one of those Verizon wireless gateway devices that give private IPs. I had to go to the Port Forwarding tab and forward Port 443 to the external IP of the Meraki MX.
I am using the AnyConnect client for VPN access which is why I use 443. If you are using the built in Windows client, I think it is 1701 or 1723.
Anyway, you are going to have to work with your ISP who gave you the device to figure out how to forward the ports needed.