All FS hardware that has both input output bus requires a software to bridge. E.g. CDU, MCP, Radio Panel. Even if you are using Saitek/Logitech device that has display, you also requires their software to interface between the sims and hardware. There's no workaround.
When a manufacturer states plug n play, it meant the device is programmed and mapped ready to be use without the additional programing or mapping knowledge such as those using FSUIPC or LINDA. For those device or own built device there's a tedious process of coding each control key to get the correct behaviour in the sims.
Flight Sim is a long journey and will have its own satisfaction when you get all different things to come together and work. It requires a lot of patience, troubleshooting skills and a passion to learn continuously. I hope you start to enjoy the journey and find great joy in the process. Cheers, have fun and fly safe!