Hello Arwen_713,
Welcome to Apple Support Communities!
I understand from your question that you keep getting a pop up telling you that low data mode is on. I can certainly help you with this!
I would start by turning on and off cellular service along with toggling low data mode once again.
View or change cellular settings on iPhone
The next step after this if this does not resolve your issue is to reach out to your carrier to see if they have a setting turned on on your line.
I hope this is able to resolve your issue. If you have any more questions or this does not resolve your issue, please reach back out on the thread!
Cheers.