Help with replacing 3 way switch with a smart 3 way switch
The DW1KD goes in the box with the switch and the funny yellow jumper

The funny yellow jumper wire was part of a scheme to convert the switch at the line/load location (the "main" location) to a single pole switch to be used with the 3-way functionality in the existing dimmer (at the "dimmer" location), given that the dimmer was installed as a switch loop from the "main" switch box where both the line and load connections are present.  However, the DW1KD has the downside that it must be on the load side of the circuit, and the current dimmer location is on the line side of the circuit.

So, you'll need to install the DW1KD at the box where the switch was, and the matching remote at the box where the dimmer was, as follows:


Remove the white wire in the top center cable from the bundle of black wires in the switch box
Dimmer black to the extant bundle of black wires and the black wire going off to the remote location (black wire, top center cable) as the remote needs always-hot and neutral to function, in addition to the communications line, instead of relying on traditional travelers
Dimmer white to the existing bundle of neutrals (it's in the top right rear of the switch box) and the white wire in the top center cable (making it a neutral instead of a switch-loop hot)
Dimmer red to the red wire going off to the fan (red wire, top left cable)
Dimmer red/yellow to the red wire going off to the remote location (red wire, top center cable)
Dimmer ground to the existing grounding pigtail
Remote black to the black wire in the remote box
Remote white to the white wire in the remote box
Remote yellow/red to the red wire in the remote box
Remote ground to the bare ground wire in the remote box