iOS 15.3 update

Start here:

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

If you have a VPN App or profiles installed, or a security App such as Norton, these may be interfering with DHCP and preventing your device from obtaining a valid IP Address for your network. The resolution is relatively simple; may only need to temporarily disable or remove the Security App and/or VPN.

DHCP is frequently used to automatically obtain an IP Address from your WiFi Router. Until your iPad has a valid IP Address to connect to your network, you will not have an internet connection. Disabling or removing VPN and/or the offending security App should allow DHCP to operate normally - allowing connection to your network and the internet.

If this resolves your issue, the VPN and/or security App can be reinstated after the connectivity problem has been resolved.

These support pages outline both the available methods by which the update can be performed - and troubleshooting should you experience difficulties:

Update your iPhone, iPad or iPod touch – Apple Support

If you can't update or restore your iPhone, iPad or iPod touch – Apple Support

If your iPhone, iPad or iPod touch won’t update - Apple Support

iOS update and restore errors – Apple Support

Be aware that your iPad may be too far behind the current version to be updated directly from iPad settings. As such, you will likely need to use a computer to perform the update. You will need access to either a PC (with iTunes installed) or a Mac (with iTunes or Finder - as appropriate for the installed version of MacOS). Ensure that your installed version of iTunes has been updated to the most recent version; the essential device drivers are included.

Download and use iTunes for Windows – Apple Support

Update to the latest version of iTunes – Apple Support

Update software on iPhone, iPad or iPod touch in iTunes on PC - Apple Support

You should note that for your iPad to install an available update, its battery must be charged to at least 50% capacity - and/or be connected to its Power Adapter. Similarly, the iPad can only be updated over a standard WiFi connection - or be updated by a PC or Mac using iTunes or Finder - over a USB connection.

Any chance, you are using an iPad with cellular option?

In that case, make sure that a valid SIM is inserted during the iOS update. If the SIM is inserted, it worked for other users, to pull out the SIM tray and put it back in again.