We have the same issue with Swedish Telia. More than one SubmitSM in the TCP packet will cause them to fail (Error Invalid Source Address).
Setting the TcpClient.NoDelay did not resolve our issue.
Note: We are not using the InetLab SMPP client (yet?), just scouting our options.

I'm arriving a bit late to the party. Seems like the link is broken, I assume @alt referenced this site?
https://docs.inetlab.com/smpp/v1/articles/async_sending.html

When it comes to window size you could do something like this
public class Sender
{
private readonly ISmppConfig _smppConfig ...