This is for Mercedes Me, the symbol is normally shown when the tracking services are enabled. Basically it is telling you that the comms module is connected to the mobile network and is pushing data to Mercedes Me for the tracking service.
When I had my 2018 C250d, I only found out what it was when I enabled the tracking service whilst sitting in the car. All other services were enabled apart from the tracking. As soon as the tracking was active this appeared on the screen in the car.
I suspect this has something to do with privacy laws in certain countries. I. E. to make occupant's aware the tracking is enabled on the car.