But connection 1 and 3 mean, that the player gets created on the server.

Try to let the client create the player entity:

if(connection == 2)
{
player = ent_create("testing.mdl",vector(100,0,0),fPlayer);
}

Else, the player does not belong to the client, but
is spawed by the server, and it would not change the client_id

"client_id:
# On a server, this variable is automatically set on all entites created by a client.
# A7.70 On a client, this variable is set to a negative value until the server confirmed the creation of a global entity and the handle becomes valid. Then client_id is set to dplay_id.
"