Q: How to I approve a new device on iCloud????

I am an overworked caregiver of a veteran living overseas. I cannot find my iCloud security code; I have my Apple ID. We have an iMac, 3 MacBooks, an iPad, and a used iPhone. The iPhone keeps threatening to stop working as it needs approval. I have approved it for both the iMac and the iPad, but it still keeps asking over and over every day at least once. Help! My husband is ill; I have to have a working phone.