Re trinary:
Last issue she used a cell phone ( and therefore WiFi/internet/whatever) to record video of jean and presumably share it with all cell phones on the planet. She was using a network to reach out to other networks.
I imagine she can’t just connect to all the sentinels without cerebro because they aren’t on a network like everyone’s phones are. She basically used the knowledge from storm’s sentinel on how to reach the other sentinels but cerebro helps her connect to them all just like the cell networks helped her broadcast that video to all cells. Just how I conceptualize it anyway.... two different things.
Also since these sentinels are in peoples brains, and cerebro tracks people with brains, well you can see where I’m going.