Leviton Smart Dimmer switch installation. Dimmer (DH6HD) works but Remote (DD00R-DZ1) light up but doesn't function

Installing in 4 way configuration using (1) DH6HD Dimmer and (2) DD00R Remotes. The dimmer works fine and the remotes LED are on but they don’t function.

Dimmer is wired (2) Blacks - Hot to BK and other black to R. White to WH and Red to YL/RD.

Remote in 3 way position is (2) Black to BK, White to WH and Red to YL/RD.

Remote in 4 way position is (2) Black to BK, white to WH and (2) Red to YL/RD.

In this photo, the 2 dimmers in the same wall gang are on different breakers. The left dimmer works fine. the right is a the remote.

Neutral were wired with pigtail to ensure all white are together to dimmer and each remote.

Sounds like the Red doesn’t truly connect the YL/RD on the dimmer to the YL/RD on the remote. Or maybe something else is connected to the Red wire, destroying the signal between the dimmer and remote. If that picture is showing a dimmer and remote next to each other, just try connecting the traveler wire between them to test functionality of the remote.

Also, why are you posting about a HomeKit dimmer in the SmartThings forum? Are you connecting to SmartThings via HomeBridge?