Is there a firewall on an iPad on iOS 10, and is it configurable? I have an iPad Air 2. I've been using a comic book reader called Bookman for years. It allows importing books over FTP on a set port (2121). It has worked for years without an issue, but ever since I updated to iOS 10.0.2, I keep getting "ECONNREFUSED - Connection refused by server" trying to connect to the iPad's IP. 

A ping to the iPad's IP works, and connecting to another app that uses port 80 works. I verified it's not a client side issue, by trying from several machines. At this point, I think something in iOS is blocking the port.

Is there a firewall, or any other configurable part that I can allow traffic through?
There is no firewall on iOS and it does not block any ports that I am aware of.

I'd guess your issue has more to do with the fact that Bookman hasn't been updated in over three years and some change in iOS 10 finally broke it. I'm actually shocked it works at all.

It sounds like you can use iTunes file sharing as an alternative, presuming that a recent iTunes update hasn't caused that to stop working with this particular app.