Xfinity no help

I have a Moto g phone that has to be restarted every time I leave my home. I noticed that my Xfinity mobile bills were doubling and sometimes tripling. Since they require you to have automatic billing I did not catch this right away cuz it was on my credit card bills. Xfinity came out and changed my router and said that could be the problem. I am on the 1G a month plan because I only use my apps when I'm in the house and only use my phone in the car to receive or make phone calls. I was told that if my Wi-Fi signal is weaker than my data signal it will choose the stronger of the two signals and will use my data even while in my house next to my router. I know does not make much sense. I was instructed to turn my mobile data off on my phone because you do not need it even when you're driving. Xfinity mobile should pick up the signal. While that has helped keep the data usage down, I still have to restart my phone whether my data is on or off every time I leave my house, out of range of the Wi-Fi. My phone will not automatically switch to Xfinity mobile from my home Wi-Fi when I am out of range. Xfinity tech support has been no help at all and have wasted countless hours trying to get this resolved. Do not see anything in the phone settings to help this problem. If anybody has suggestions I would appreciate it.