Arduino Nano as DMX decoder

Using Arduino Project Guidance
1

Hi,
I build an LED-Staff using ws2812 LEDs and an Arduino Nano as controller.

Now I wanted to improve my build by using the Arduino as DMX decoder, because I want the staff to be part of a light show.

I used this Video as reference an basically copied the recieving side of it. To test the whole build I connected the MAX485 interface to a normal DMX controller but nothing happened.
The red "data recieved" LED on the Arduino lights up though, so I guess something is happening.

When I upload different code to test the LEDs it works so maby the Code is the problem, but I only understand it partially.

Code for using the Arduino as Fixture

Another challenge I'm facing is the limited space because it's supposed to fit inside the staff (hence the Arduino Nano).

I would really really appreciate any help (and please excuse my poor english).