Internet data use

Five months in on new TMobile internet service and loving it but have questions on why data use is showing so high.  Situation:  Nokia gateway.  I have a home security camera NVR connected to one of the ethernet ports on the gateway.  On the other available connector on the gateway, I have an ethernet cable that runs to a 16 port non managed switch to which 7 IP cameras are connected.  NVR records on these cameras 24/7 continuous, and there is no cloud upload or storage or anything like that.  This NVR system was new the same time as moving to this internet service so I don't have historical to compare before and after data use.  Would this configuration of the camera system cause high internet data usage for some reason?  The only other devices using internet are a couple smart wifi light bulbs, smart thermostat, and a smart tv that is powered off at night.  Computer, ipads, etc. are all powered off all night, but I still see high data numbers in the tmobile app under usage that displays the two hour time segments.  For example, in consistent 2-hour increments in the middle of the night, even if no one home, data usage is still in the 15-30mb every 2 hour range.  This seems high for how I understand what the security system is doing.  I would think data usage would only spike if camera system was being accessed remotely.  Is it possible that the router part of the gateway is measuring network data transfer from the cameras to the dvr since it is going through the router?  Router admin doesn't show which devices are using the data specifically, just says how many devices connected.  I know I have unlimited data, but using over 200-250mb per month seems way high for what I am doing.  I'm not a network expert or anything close, so hoping someone may have experienced similar or have ideas.