[BC66] QLWNOTIFY failed after PSM

That looks very much like a NAT.

RFC 7252 - The Constrained Application Protocol (CoAP) defines a protocol endpoint by “With no security, the endpoint is solely identified by an IP address and a UDP port number.”, so the change of the port by the NAT is the root-cause.

Follow the links to the Leshan wiki and study the information there.