how to re-add a device i removed on find my iPhone on my iPhone?

Hi,

From https://www.igeeksblog.com/how-to-add-or-remove-a-device-from-find-my-iphone/

Adding a Device

• To add an iPhone or iPad to “Find My iPhone” (iCloud), all you have to do is sign in to the iCloud account from your iDevice and enable the switch for feature. Here are the steps:
• Open Settings
• Tap on iCloud
• If you aren't signed in already, type your Apple ID and password (your iCloud credentials basically)
• Now, turn on the toggle for “Find My iPhone”

That's it. You will be prompted to turn on location services in case that's not active. If you have any restrictions placed on what apps can access location services, you might have to remove them, especially if the restriction is on Find My iPhone. Try checking that from Settings → General → Restrictions → Location Services (under Privacy) for iOS 6 and Settings → Privacy → Location Services (for iOS 7).

Did you try downloading a previously purchased app, song or book to the device in order to associate it with your Apple ID?

When you turned Find my iPhone on again, did that Setting stick? In other words, you you see that the button is green? If so, did you try turning it off, reset the phone and then turn it on again? Did you try signing into iCloud.com on a computer, log into Find my iPhone and see if it shows up that way?

You should be able to sign into iCloud with your Apple ID and password and then turn on Find my iPhone again. What happened when you tried?

Did you try to restart both devices ?