Have you tried further locking down your WiFi with the extra security setting that only accepts computers with the MAC addresses that you have specified? I forgo what this feature is called, “MAC Addressing” maybe?
About the data monitor, what operating systems are you using on your computer? Windows 8.1 and Windows 10 have built in Internet data usage monitors. I am looking at mine on Windows 10 right now which even breaks down the exact data usage individual apps use.
For Android and iOS, aren’t there various data meta apps?