Sunnata LED+ Dimmer - need help please!

Hi, I'm trying to install the Sunnata LED+ STCL-153MR-WH in a configuration where ceiling lights are controlled from 3 switches. The first switch is a regular mechanical 3-way, the second is a 4-way, and the third switch is what I'm trying to replace with the Sunnata dimmer.

There is a currently a lutron dimmer in that position and all the switches work fine to control the lights. When I replace the current dimmer with the Sunnata, none of the switches work and the Sunnata doesn't even light up.

The wiring is the same for the current dimmer as it is for the Sunnata.

Common to black on the Sunnata, green to ground, and then the black and red in the box (travelers from the 4 way) to the blue and red on the Sunnata.

What am I doing wrong? I've wired and re-wired this thing so many times, I'm out of ideas.

Any help would be very much appreciated!

Thank you.

Because the Sunnata dimmer is a digital control when it is installed in a 4-way application the remaining mechanical switches must be removed and replaced with the Sunnata accessory switches (model # ST-AS). Here is a wiring diagram for the STCL-153M and 2 ST-AS' in a 4-way: https://www.lutron.com/TechnicalDocu...93_Page_84.pdf

Alternatively, you could replace the STCL-153M with a DVCL-153P as this is an analog control and will work with the existing mechanical switches. This dimmer wires as follows: black wire of Diva dimmer to common wire in wallbox, red wire of Diva to one traveler wire, and red/white stripe wire of Diva to remaining traveler wire.