What type of router do you have? The brand & model is important to know!!!

The firewall issue is coming from somewhere. Either the router isn't handling UPnP properly, which means either rebooting/starting the device or checking the reliability of the device. Some devices don't work reliably in UPnP mode, & may give inconsistent results for some odd computers on the network. Some may take some time to kick properly into action whilst in UPnP mode.