The user-defined DERP server does not take effect

the terminal displays the following error message

The client closed the connection and/or had the connection blocked in the middle of negotiating the TLS setup.

For a server on the Internet: this happens all the time. Randos constantly probe your https port, and indeed every port on the system. That is likely not even your device making the connection, there are frequent probes of all IP addresses known to be owned by one of the large cloud providers.