Cellular data usage wont update anymore

That is a setting unique to T-Mobile phones. Each carrier can add specific settings there, and I don't have that on my AT&T phone. That would also explain why you do not see the Reset Statistics command I described. I suggest that you contact T-Mobile and ask them about this. Only they could answer that question.

What makes you believe that this is data usage from December 2018? That usage does not reset on its own, you have to reset it manually. Is the December date you are referring to that last time it was reset? If so, that is when you manually reset the data, and it will continue to just keep adding daily until you reset it again.

Now you have me confused. I'm not sure where you are seeing Usage. I'm referring to going to Settings>Cellular>then scrolling to the bottom of that page after the listing of all apps and you will see Reset Statistics. The amount of usage for each app is listed at that app. Could you describe where you are going? Is this an app that is specific to Sprint? If that is the case, that would be a question you would need to address to them.

Chris, congrats on reaching >110K :-)

Giulio

Thank you. You seem to be doing pretty well for yourself as well.

I try to do my best, consistent with work commitments.

Have a nice Sunday :-)

Giulio

You're welcome. Hope you are able to get it straightened it out with them.