Hi Mark
On any iDevice, Go into Settings - from here it depends on your version of iOS:
but if it's iOS 11.x - Tap on your name at the top, then tap on iCloud and under APPS USING ICLOUD, scroll down till you see Safari and turn it on.
It is, naturally, essential that on your Mac, you have Safari as one of the apps to be synced via iCloud. On your Mac, it's System Preferences > iCloud and on the list presented for syncing, make sure Safari is checked.
Ian