Phone does not rejoin wi-fi

For well over a year now we have had a persistent problem. My wife’s phone does not rejoin wifi automatically when it drops it. As a consequence she over-uses the data on her cellphone contract.

What makes it more bizarre is as follows:

1. We are on our second wireless router and the problem was carried over.
2. No other Apple device suffers.
3. She has had three iPhones in the period, all with the same problem but all restored from the same back up. One iPhone SE and two separate iPhone XS.
4. Resetting network settings does not solve.

This has been frustrating to say the least. She is not tech savvy so it is difficult for me to gain more data to understand how it happens. But I have seen it many times with my own eyes. The phone sails on using a 4g network when a good WiFi network for which it knows the password sits unused.