You must use Bash as your shell. Because /dev/tcp is a magic provided by Bash.
$ exec 5<>/dev/tcp/{server_host}/{tcp_port}
$ echo '{"jsonrpc":"2.0","id":"1","method":"server.version","params":[]}' >&5; cat <&5
You'll get the result like this.
{"jsonrpc": "2.0", "id": "1", "result": "ElectrumX 1.0.15"}