iOS 15 update

Start here >>> If your iPhone, iPad, or iPod touch won’t connect to a Wi-Fi network - Apple Support

Also verify that the date/time is correctly set - as a substantial error can impact network connectivity. If the Date, Time or Timezone are incorrect, it may be necessary to temporarily manually configure them from settings:

Settings > General > Date & Time

I had this issue too…. my VPN was blocking wifi connnectin to internet (even though the iPad said I was connected). Do you have a VPN connected? If so, I fixed by issue by deleting the VPN - once I did that, I was able to connect Wi-Fi again.