Hey,
IP forwarding is enabled by default due to the way balenaEngine handles port mapping. It’s basically a NAT network into the docker bridge controlled by iptables. These rules are handled by balenaEngine as would be the case with Docker.
I am not sure I follow your use-case exactly, but regardless you can run a container in the host network with privileged enabled and you can manipulate the IPTables rules. Whilst IP forwarding is enabled, the rules have to allow the traffic in the first instance, so I would start there and see why the traffic is blocked.