My iPhone keeps telling me my cellular data is off?

I have an iPhone 6 and I haven’t used cellular data, only Wi-Fi. And every night, only at night while I’m browsing, that tab or whatever it is called keeps popping up and saying that my cellular data is off. I don’t understand why do I need to know that multiple times and after appearing a couple of times it turns my Wi-Fi off!

I don’t know what to do. It’s really annoying.. Some people said to turn off Bluetooth or something with the chip, I don’t know, but my Bluetooth is never on because it wastes my battery for nothing. Not sure what to do, any help appreciated.

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