I pad

Start here:

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

…and here:

Recommended settings for Wi-Fi routers and access points - Apple Support

If configured correctly - and your WiFi Passcode is still being rejected, a restart of your WiFi Router may resolve the problem.

To restart you home router, disconnect power - then wait at least 30 seconds before restarting. When you router has rebooted (which may take several minutes), force-restart your iPad:

• On an iPad with a Home button: Press and hold the top button and the Home button at the same time. When the Apple logo appears, release both buttons.
• On an iPad with Face ID: Press and quickly release the volume up button, press and quickly release the volume down button, then press and hold the top button. When the Apple logo appears, release the button.

If you are using a wireless, WiFi MODEM/router box to access your Internet over an iSP and your iPhone can get online from this box, using this home WiFi network, I suggest you check your WiFi settings, in the Settings app on your iPad, and check to make sure the WiFi device login name is the same as it is on your iPhone and make sure your iPad is using the same WiFi device password.

On the iPad, You may need to tap on your WiFi device's name and then tap “Forget this Device” and attempt to setup your Wifi box/device name and password on your iPad, aga

Also, you may need to access your MODEM/router box internal software>

You can accesss your WiFi MODEM/router box software through a Web browser.

You will need to look for your MODEM/router's setup/instruction bookket that will explain how to access the MODEM/router software and its settings .

You may want to look at this link below, from Apple, as well.

Recommended settings for Wi-Fi routers and access points – Apple Support (UK)

Got VPN?

Try disabling it or, temporarily deleting it from your iPad.

Then try re-establishing your WiFi connection.

If you choose, re-download the VPN and re-enable it and see if your WiFi connection stays connected.

Best of Luck to You!

How is your iPhone getting onto the Interenet, exactly?

Just Cellular service?

Home wireless, WiFi MODEM/router network box using a ISP ( Home Internet Service Provider )?

That'll give all of us users here a clue to how to get your iPad onto the Internet.