Data Usage due to Poor Coverage

Over the last few month's i'm constantly having my trouble with wifi coverage at home and at work. My phone always seems to be buffering or not able to download things that are texted to me. My cellular usage doubled and i have no other explanation besides this. I haven't changed what i use my phone for or downloaded any new apps. I contact Verizon about it but, was just told to increase to unlimited data. I never used to get an error message that would pop up when i was using an app asking if i want to keep trying wifi or use cellular data and now it happens daily. I was also told I could be using more data if i haven't upgraded to the newest apple ios but i do that as they become available. Has anyone else seen a large increase in data usage with no real explanation this year?