How to track monthly internet useage?

[Richard Owlett <rowlett@access.net>]

I use Debian 12.8 with Mate 1.26.0 as desktop.
My primary browser is SeaMonkey 2.53.21 .
My internet connection was sold by T-Mobile as a "WiFi Hotspot".
[I have the WiFi disabled thus essentially using it as a high speed modem]
My service has a LOW data cap which I occasionally exceed.

I wish to log essentially daily cumulative totals for my usage.

Mate's System Monitor will give me _current_ usage but I'm looking for a 
scriptable tool to essentially log daily usage for a month at a time.

What should I search for?

TIA