Can the notify When Left Behind feature be set up remotely for my iPhone 7?

No. All Find My features and settings can only be setup or changed if you have the device physically in your hands.

Notify when left behind does not however work for an iPhone, because you would have no device with you at the time you left it behind to be notified on, so it does not actually make sense to have it active for your iPhone. The Notification appears soon after having left it behind on your iPhone. If you don’ have the iPhone with you, then you have nowhere to get the notification on. There’s no way to magically notify you until you get home or can access another device that is logged in to your Apple ID, which would render the notification pointless.