Matter over Thread requires IPv6 on your local network, so if you can’t pair the IKEA Kajplats bulb that might be why.

I didn’t have IPv6 enabled, and my ISP doesn’t assign IPv6, so I used unique-local-ipv6.com to get a unique prefix. Configured it as static in my local network setup, alongside the IPv4 configuration, on my UniFi Dream Machine and then all my bulbs connected.

Now my niece is learning to pronounce English colors as she cycles my apartment through every shade of the rainbow 😁