Some access points have a feature to hide or not activate broadcast SSID, one of which is TP-LINK TL-MR3420.
How to Hide SSID
* Wireless->Wireless Settings.
* Uncheck Enable SSID Broadcast.
How to Connect Wireless to hidden SSID
* In this tutorial I use the Ubuntu distro to connect wirelessly to an access point with a hidden SSID.
* Click the network icon->Connect to Hidden Network
* Connection: New
* Network name: SSID name
* Wi-Fi security: type of security used
* Password: wifi password
* Click Connect.