Wifi assist

First try to toggle Cellular Data ON

1. Goto Settings > Cellular
2. The first option: Cellular Data. Toggle ON
3. Now, scroll Down on the same page to Wi-Fi Assist. It should not be greyed out and you can turn it OFF

Check your Screen Time restrictions --> Use parental controls on your child's iPhone, iPad, and iPod touch - Apple Support

GO to "Allow changes to other settings and features" and check Cellular Data Changes > Allow