You see no output - that means the connection is not established.
I don’t know what answers your connection on the other side but it is no CMK agent.
Sample connection to 6556 looks like this
telnet 192.168.188.77 6556
Trying 192.168.188.77...
Connected to 192.168.188.77.
Escape character is '^]'.
<<<check_mk>>>
Version: 2.0.0p1
BuildDate: Mar 16 2021
AgentOS: windows
followed by a huge amount of text