Thank you for the detailed reply. I'm happy to ignore the HomeKit Barcodes for now as you are right it sounds painful to work with, the app idea was originally aimed at Matter Devices anyway. I can give the HomeKit devices manual pairing code and advise the user to do it manually in the Home app, so no big deal, I'll concentrate on Matter Devices.

Cables and the router have nothing to do with this, it worked great up till IOS 15. I have enterprise grade gear with all the right connectivity and it's still trial and error as to whether it works or not. I would not recommend anyone go this direction when trying to solve this as it's likely money down the drain.