Fing a great app to monitor your wireless connection

I've noticed that our internet data usage has went up this past few months. My nieces in the house are just surfing the net, watching videos on youtube, watching movies on netflix, and playing minecraft. As far as I'm concerned we don't downlaod anything. I downloaded an app cal Fing. It's avaiable for the Iphone and Android phones. I could monitor from there those devices that are connecting to my router. I noticed that there was one particular ip address that I cannot identify. So I tried blocking that ip add but I'm not able to do it. so ichange my  SSID and password. I selected a WPA2-Personal Encryption. Would this hold up and do the job? Well thans to Fing I am able to monitor properly those devices that connect to our router.