The nDroid app communicates with the nDroid Service using HTTP. As of v1.7.x of nDroid, two ports are used (one for the main service and one for the streaming service).

By default the main service listens on port 8790 and the streaming service listens on 8791. Please make sure both of these ports are open in any firewall you use.

If you change the main service port to another number, the streaming service will use that number + 1. For example, if you change the main service to use port 9000 then the streaming service will use 9001. Make sure to follow that pattern when changing firewall rules.