Not if your router is set up correctly. If a device correctly knocks it gets temporarily added to a whitelist you don’t just open the port.
IF you want to put custom firmware on your business router to allow port knocking, I guess that’s fine. But there are still plenty of ways to misconfigure RDP and leave the protocol open to attack.
Much simpler and less error prone to just use a stock router with standard OpenVPN - you get a more likely to be correct setup and also a lot more functionality than ‘just RDP’ if OP needs it.
It depends on OP’s router many support knocking out of the box but it does depend on knowledge and situation.