Thanks for your help!
I just wanted to give a quick update: the issue is resolved.
It turned out that the problem was caused by the Shelly binding version I was using. After updating to:
Shelly Binding 5.3.0.202607282233
my Shelly Plus 1 (FW 1.7.5) immediately initialized correctly and the Configuration Pending state disappeared. No changes to CoIoT, RPC over UDP, or authentication were required — the newer binding handled everything properly.
So the binding update was the key.
Thanks again for pointing me in the right direction!