After looking at the other answers on the site, my issue persists.
I'm using a ZTE Sonata 2, and I registered with a Pokémon Trainer Club account, not a Google account. Yesterday, the game worked as normal. Now it works only when I'm connected to my home wifi. It doesn't work on other wifi hotspots- only our home wifi. I have plenty of data left, which works for any other app.
I can open the game on data or on other wifi. It lets me log in. It brings me to the map and lets me walk around. But no Pokémon appear in the tracker, and there are no gyms or Pokéstops. (There's a gym and two Pokéstops visible from my house with wifi. They're gone when I connect with data.)
If I connect with our home wifi and then switch to data, the Pokéstops and gym are still visible on the map, but can't be used when I go to them, and Pokémon disappear from the tracker. Pokémon do not reappear if I come across a wifi hotspot.
I have looked at various other questions on the site regarding Pokémon Go and data, including the most popular one. (Why can't I connect to Pokemon Go via mobile data?) Every question on the topic seems to be marked as a duplicate of that, but that question didn't help me. Here are a few differences between my question and that one.
A. I can log in from data, I just can't play.
B. I can't play at all on data, even if I start from wifi.
C. Finding another wifi hotspot doesn't help.
D. I'm not using a Google account, I'm using a Pokémon Trainer Club account. (All of the answers on that question related to this.)
E. It wasn't like this yesterday- only today. (The other question technically doesn't state anything contrary, but that's the impression I got.)
I've tried restarting the app and phone both, and also clearing the app data on my phone. None of them helped. I found one question that seemed similar to my issue (https://gaming.stackexchange.com/questions/280084/pokemon-go-cant-see-pokestops-gyms-etc-without-wifi?noredirect=1&lq=1) but it was marked as a duplicate of the former and left unanswered.