Phone shuts off in 3g

I have a Moto Z2 Play with Android 8.0.0, about 3 years old. Whenever I go out of range of 4g (to 3g), the battery goes from 100% to 80 or 70 in a couple minutes and then to single digits very quickly and the phone shuts down. The phone will not turn back on in this state. When I charge it, it returns to 70/80 immediately and it will turn back on. This never happens in 4g. What is the problem? Is the battery just faulty? --- follow up --- I attached a power bank and went out. Here is what happened. It lasted longer, perhaps an hour and then went to single digits and then turned off. I then restarted the phone with the power bank and it was quickly back up to full power. What does this tell us?