Samsung Galaxy S5 — Network issue

Hi everyone,

I am having issues connecting to a network with my (unlocked) S5. My carrier in the US is Ting, and it was working fine until I recently traveled to Nigeria and used Airtel while I was there. The Airtel employee made some changes in the phone's settings when I purchased the sim card, and it worked fine.

Now back in the US, I just got a sim card replacement from Ting (I had lost the old one), but I can't get any service on the S5, although I activated the sim and it works fine in my other phone (an old Moto E).

I've tried resetting the network in the Settings menu, but nothing changes. When I turn the phone on, or wake it up after it goes to sleep, I see "Ting" appear in the top left corner for a second, then it disappears, and I don't get any service. The Ting customer service suggested a factory reset, but is there something else I should try before I do that?

Thanks a lot!