Is there any function that can monitor and display or better, block cellular data usage at a specified total per month?
This is a basic feature of many cellular hotspots to monitor total data usage on the LTE connection, and reset the counter on the specified monthly date. Good ones will show this on the display and block traffic on reaching a usage threshold. Many users have a limited or fixed monthly data plan and it is very important not to incur overage.
There is no such functions now.
It is useful to add though.
Being that most of these routers are used with cellular connections, (USB modem, tethering, etc) this should be a requirement - as most cell data is metered. At least implement a way of manually tracking/resetting a data counter such as the one in luci under the Interface tab. Ideally with the ability to persist the count through interface up/down events such as disconnecting the cellular device or reboots. Even better would be a new function of the GL.iNet GUI, with the ability to push notifications, and set the monthly data cap and reset date.