Hello,
Indeed, this seems like a firewall issue. Could you try creating a separate network interface, binding the connection to it, and then creating a separate firewall zone for this interface and allowing the input, output, and forwarding? Then this zone could be experimented with (allowing forwarding from different zones, etc.)
I'm not quite sure how this package is implemented and do not really have a way to test it. Perhaps someone more experienced with this package will stumble upon this thread and will be able to help out.
If the question about Netmaker is brought up by our RnD team, I'll inform them, that there is interest in it.
Best regards,
DaumantasG