Sign in to the same Apple ID on both iPhone and iPad
Open Settings on iPhone
Tap your name
Tap iCloud
Turn on Contacts
Open Settings on iPad
Tap your name
Tap iCloud
Turn on Contacts
If prompted, choose Merge
Connect both devices to Wi-Fi
Wait a few minutes for contacts to sync
Open the Contacts app on iPad to check the synced contacts
If contacts do not appear, restart both devices
Make sure Contacts sync is enabled under iCloud on both devices
Check that iCloud storage is not full
Update both devices to the latest iOS or iPadOS version