Hello, friend!
I don't understand what you mean by "open". Generally, ports on a device are enabled by default.
Port 3389 is the service port for Windows 2000 (2003) Server Remote Desktop. Port 3389 of the ONT (HG8145V5) has no service. You can map port 3389 of the PC to port 3389 of the ONT through port mapping, and then implement service mapping.
For details about how to configure port mapping for the HG8145V5, click:
How to configure Huawei ONT port mapping
Thanks!