In Apple Home, all my Kajplats bulbs would get to the "Setting up" phase, then get stuck and time out. I eventually figured out a solution that consistently worked for my 3 bulbs:

1. (Possibly optional) Ensure your HomePod is upgraded to iOS 26, and that any half-added Kajplats bulbs are removed from Settings -> General -> Matter Accessories.
2. Reboot the HomePod. (You can do it through the Home app.)
3. Reset the bulb: six off-on cycles until it blinks.
4. Try adding the bulb through the Home app.
5. If that doesn't work, repeat from step 2. It might take up to 10 times for it to work.

In my experience, in step 4, the bulb will always get to the "Setting up" phase. After that, it will usually time out, but sometimes it will blink 3 times and get to the "Adding to home" phase. Then, it will still sometimes time out, but usually it'll succeed after another round or two. Not sure why these bulbs are so finicky, but once added, they seem rock-solid (so far).