Data usage frustration

We switched from being long time AT&T users to Cox in mid August.   The first few days everything was fine.  But literally after a week of service we began getting daily usage monitor alerts.  We have since been way over our allotted monthly 1TB of data.  After calling Cox, having a rep come out and check the setup, our usage dropped back to normal levels.  But we ended up at 1,873gb used for the first monthly billing cycle!  Then,  one the 2nd day of the new billing cycle our daily usage mysteriously shot up again to wll over 75gb a day and into the 80's gb per day usage.  All with no additional devices,  no changes to devices or usage, nothing to cause new peaks or anything of that nature.  We have changed the password on the router and the wifi and I am monitoring the devices connected to watch for any intruders, malware, viruses, or anyone stealing the signal but nothing happening in those regards.  The concerning and puzzling part is that we have the exact same devices as we had setup with AT&T and our daily usage isn't any different than anything we were doing before.  We never once in the last 6 years went over our monthly allotted 1TB with AT&T.  Nothing different in monthly data allotment or devices, or usage.   Cox won't help basically because we purchased a 3rd party Netgear router.  

Anyone have same experiences, knowledge, or ideas of anything I can check or do to figure out what is driving this huge difference in data usage? We had switched recently from AT&T to Cox because the service was expected to be better with less interruptions on Cox  But if this continues we will have to switch back to AT&T,