I am thinking there might be two possible issues going on:
1) on the phone the software that activates the IP over Cell / Data may have a bug that is not re-enabling it all the time.
2) the carrier might have a problem with the part of that they do.
internet over cell works a bit different than WiFI and I have worked with some of the logic on older windows mobile devices where we added code to test and force the device to "re-connect" at times.
by default they time out the ip part of the cell connection when they do not see it as "active" for a while. kind of like how dialup used to have a time out on the profile.