I use cellular internet, too. The first company's service worked for a couple of years and then it didn't work as well. They kept trying to blame it on me or my equipment, the leaves, tourists.... I changed to a different provider and it works again.
I'm not sure but I think the first company was no longer the "A" on the tower; their lease ran out or whatever. The second company (that works) is on the same tower. So you may get better reception with a different provider.
Then again, I can see the tower from my house, if your tower is further away it may not make much difference.