iPhone 11 using data while showing connected to WiFi

My son has an iPhone 11. He will be at home and bars showing he is on WiFi but then get an out of data alert. Usage shows the phone has been using data while supposedly on WiFi. Have been on the phone with Verizon several times and adjusted various settings to fix this but nothing works. We are constantly running out of data. Verizon now tells me this could be because of a bad antenna in the phone. Has anyone heard of this issue with the antenna or had this happen to them?