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Thank you for provided information. To get a notification when router reaches certain mobile data limit, you need to configure Mobile Data Limit which can be found by connecting to the device WebUI and navigating to Network -> Interfaces -> <Select your mobile interface and edit it> and there you will find the settings for Mobile Data Limit.
More information: https://wiki.teltonika-networks.com/view/RUT955_Interfaces#Mobile_Data_Limit
Furthermore, I strongly recommend upgrading device firmware version to the latest one: https://wiki.teltonika-networks.com/view/RUT955_Firmware_Downloads