since ios15 my ipad no longer syncs with iphone

After ANY iOS/iPadOS upgrade/update, you should ALWAYS perform a simple hard reset of your Apple mobile device.

For older iPad models with a Home button, do a simple hard reset of your iPad by holding down both the Home and sleep/wake/power button simultaneously until your iPad goes to black and restarts with Apple logo, then release the buttons.

OR

For much newer, larger screen iPad models, without a Home button, try the new simple hard reset procedure for the new 2018/2020/2021 iPad Pros and the 2020 iPad Air 4 models.

1. Press and release the volume up button.

2. Then immediately press and release the volume down button.

3. Then, press and hold the single sleep/wake/power button You will see the Slide to Power off button, but keep on holding down the power button until you see the Apple logo, then let it go.

Once you have performed all the steps, wait for a few seconds and your 2018 iPad Pro will boot up completely to the iPad Pro’s Lock screen.

Do the same applicable hard reset procedures to your iPhone.

You may need to re-sync both your iPhone and iPad, again to establish proper sync-ing.

Best of Luck to You!