I had similar issues but managed to install my GRILLPLATS (and other IKEA Matter devices by using an old-ish spare-phone. (described in this thread: Thread network question )
I don’t use IPv6 at all (disabled in router) so I don’t think that matters.
Short version:
I took my old-ish phone, did factory reset, and only installed & configured Homey app.
Keeping phone, Homey and GRILLPLATS plugs really close together during config.
It took a couple of tries but all the sudden it worked just fine and by repeating on all “hard to config” devices I have all my IKEA stuff properly installed !
