Thanks for your prompt response. I do have Matter server app, the Thread integration and the Open Thread Border Router app installed
and I had already toggled Use for Android + iOS credentials. I even reset the border router a couple of times, yet I can’t seem to be able to send the key to my iPhone.
You are right, I don’t have a double NAT. The error message that I see is exactly the same as the OP, but the root cause may differ. Will create a new thread then.