All parts of an autonomous system break. Safety engineering is measuring those breakages, and designing a system that is safe when it inevitably breaks.
If you're the operator, you can choose to only drive on routes that should have sufficient connectivity. If your remote driver is only issuing high level commands (i.e. not responsible for safety) latency starts to stop mattering so much.
The problem here is the availability bias - you've seen your phone fail so you know that telecom links can fail. As a layperson you might not think about how the rest of the system suffers from the same limitations - but they do. You engineer around them.
So it is more than sufficient if you can show that AVs - on average - cause less harm than human-operated vehicles.
To your specific question: when the AV loses connection, it would do the same as a human driver when a tire is punctured: turn on warning lights, slow down, and stop at the roadside. Like in other car failure situations, that might cause an accident in some cases. However, that is fine, as long as it is rare enough.
(disclaimer: I'm not working in AV tech, I don't know if current AV technology handles this case as imagined)