Can't connect via RDP to VirtualBox VM from local network

You have to decide which RDP server you want to use.

Windows has a built-in RDP feature. This feature, which is available in Pro editions of Windows, can be enabled in Windows settings. You're connecting to it using VM's IP. You have to provide Windows user's credentials and Windows will log others out when you do so.

VirtualBox also has a RDP feature. It works for all kinds of guests, not just Windows and not just Pro editions, because VBox itself becomes the RDP server. It's configured in VBox's settings and uses configured credentials, not Windows credentials. In this case you have to connect to host's IP and Windows guest doesn't know you're connecting.

These two are completely independent and not mutually exclusive.

If Windows's built-in RDP doesn't work, you can give VirtualBox's one a try if it works for your setup.