Who is your carrier?
Some have better service than others.
Did you set a data limit?
Or a data warning?
I know that on an older device, I did set up both a warning and a limit so that I would not use more than my monthly allotment.
But it screwed things up, and would tell me that I was out of data long before I actually was.
I had to eliminate both the warning and the limit, and then it worked fine.
My newer devices are not that way, I can set the warning and limit, and they work as they should.
Check and see if there is a warning and or limit set up on the data in your device, and if so turn them off.
Also, your device is rather old, and it is quite possible that your carrier has changed technology to the point where such a device can no longer use data over cellular.