To change the default RDP port to a machine, edit the following Windows registry setting
and reboot to apply the changes.

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Terminal Server\WinStations\RDP-Tcp
Port Number (DWORD / Decimal)

Don't forget to configure port forwarding on your router and enable this port on your firewall.

From the WIndows Network Adaptor properties set your machine to have a fixed IP address.

From your Router's control panel, browse to the DHCP page and change the range of available IP addresses to
exclude this one, so that you don't create a conflict from another device.

Go into the Router's NAT / Port Forwarding page and set the TCP port used in your registry to point to your
machine's fixed IP address.