So I recently lost an iPhone 6s Plus, and ordered a new iPhone 7 plus through the insurance, once the 7 arrived, I restored it via iTunes from a previous backup, now here's the issue. It is logged into my current Apple ID and iCloud accounts, and it is asking me to sign into the accounts to confirm, but they have a two way authentication that sends a 6 digit code to your main device, so essentially it should be getting sent to the new 7, but I receive no message
The first piece of the puzzle for you is to get your list of trusted devices and trusted phone numbers updated.
Be aware there is an older two-step process and a newer two-factor process so I usually print out the article and keep notes when helping someone so we don’t go down the wrong path for the other process.
The FAQ at the bottom of the article might cover your exact situation, so start there.