iphone 6s wifi

I have an iphone6s.

After the upgrade to 11.2.2. when the phone is connected to the wifi and goes to stanby after I reactivate it (either after a few minutes or many) the iphone no longer connected to the wifi network but is in 4g. To re-establish the connection to the wifi I have to redo the procedure from general settings> wifi and choose my network.

How can I solve the problem?

thanks

Erica