I had this happen to me yesterday with my Pixel 3. It said No Internet and then small window popped up saying "you have exceeded your allowed data usage - you may hit resume but you will be charged for any data over". So, I was very upset because I have unlimited data. Finally discovered that it had nothing to do with my "unlimited" plan. There is a setting on the phone that had been turned on with a limit set and that is why this occurred.
So if this happens to you - go to settings search for "data" when that screen comes up if "Set Data Limit is switched to "on" turn it off. I discovered this AFTER I complained to Verizon asking them why I was being charged when I was on the Unlimited Plan. 🙂 I'm sure this is a convenient setting when you have children on your plan and you want to have control of their usage limits.