Make sure both devices are on the same network. There can be a situation where device 1 (your computer) is utilizing a WiFi network while device 2 (your phone) is not connected to the same WiFi network but is using it's cellular network. Make sure both devices are on the same WiFI or LAN network.
Check your firewall settings on your PC, it may be blocking incoming and/or outgoing traffic. Be sure to enable connections to and from URLSender