the SDLNet_TCP_Send() is designed for the client only (according to the documentation).
Perhaps this is confusing:
I prefer the term "peer" for this purpose, since it doesn't confuse terminology or imply direction.
... such as the client disconnecting.
But no, client here means "the thing on the other side of TCP connection", nothing more.
The server cannot send information, and the client can't receive information.
If you can send in one direction, you can send in other. But hard to say without more detail.
Have you tried first going through the SDLNet tutorial?