Netgear m1 screen over view

I’ve just purchased this hotspot and I’m not sure what the 2nd half of the circle is on the front of the screen. I know the circle half to the right is the battery percentage. But what is the 2nd half of the circle to the left ?

What I’m trying to ascertain is why when I sometimes use it this dial is lit all the way up to the top and sometimes when I use it the dial only has a tiny spot of colour.