I had the same problem as everyone else with my data (3gb) used up the 1st or 2nd week. I have gone through this for over a year and have called VeriOn to no avail! I stopped by best buy a spoke with one of the cellular reps beacuase I was ready to drop Big Red for AT&T. However, the rep said it was not enough to turn off the cellular under wifi but the soulution was to cut of WIFI ASSIT which is totally different. In additon to that I turned of a lot of functions that I only use on my secure home network anyway. I am monitoring my usage and will let you know the outcome after my next month allotment. I paid 10.00 for 1 more GB and today it shows I have 952 mg left which shows that my problem is solved, I think........
Wi-Fi Assist is a good one to try. I've personally had Wi-Fi Assist on since it rolled out and never seem to have any challenges. I routinely use my device in my carport and workshop next to my house. Although the Wi-Fi signal is not as strong in my workshop it rarely ever flips to cellular. I've seen some people swear by this causing challenges so I think your on to something. It really depends on how strong your Wi-Fi signal is in your home / apartment.
Other options would include turning off cellular for apps that you don't want to use cellular. Or going to a higher plan. I've had a plan that includes more data than I need and now with it rolling to the next month I feel like I can use it up in the next cycle.
Settings > Scroll down to the app in question > toggle off cellular data for the app.