Why can't my phone connect to cell network data?
Turns out this wasn't a problem with the phone. I called Ting customer support and apparently all that happened was I hit 5 GB of data usage, so they shut off my data automatically to prevent an unexpectedly huge bill at the end of the billing cycle.

According to the support person, you can self-service this by logging into Ting and looking at your device settings - the "can use data" option should be off. Turn it on and you'll be good to go.