Enabling UPnP on the router isn't going to help except for those cases where your PC and/or XBOX need to open ports on the router to allow inbound traffic to their respective platforms. At the moment this sounds more like a firewall issue. Perhaps the Windows firewall is preventing access to your PC from the XBOX? Perhaps you're using third party anti-malware that's preventing access? I would just shutdown any and all firewalls, anti-malware, etc., and see if things change.