This isn't an iPhone 7 or Verizon issue. The iPhone switching to cellular data when wifi signals are weak is a feature of iOS introduced in ios9. It is there purposefully to assist weak wifi signals but sometimes doesn't seem to work the way intended. Wifi assist can be turned off so that when on wifi it will not use cellular data at all, even if the wifi connection is weak.
To enable or disable this, open setting then go to cellular, then scroll all the way down to the bottom and you should see an option for wifi assist.