This snippet checks if the router is available to port forward using Godot's RPC system

UPNP.gd

extends Node2D

func _ready():
    var port = 12345
    var upnp = UPNP.new()
    upnp.discover()
    for i in range(upnp.get_device_count()):
        var upnp_device = upnp.get_device(i)
        print('device[',i,']:')
        print('- igd_our_addr: ', upnp_device.igd_our_addr)
        print('- igd_service_type: ', upnp_device.igd_service_type)
        print('- igd_status: ', upnp_device.igd_status)
        print('- service_type: ', upnp_device.service_type)
    print('gateway:', upnp.get_gateway())
    if upnp.get_gateway() and upnp.get_gateway().is_valid_gateway():
        var res_udp = upnp.add_port_mapping(port, port, 'Godot', 'UDP')
        var res_tcp = upnp.add_port_mapping(port, port, 'Godot', 'TCP')
        print('res_udp:', res_udp)
        print('res_tcp:', res_tcp)
    else:
        print('no valid gateway')