It can only be erased with FindMy remotely with a network connection that will not be possible if the device remains in Airplane mode. All you can do is put in the request to erase with Find My and it will be erased if it can connect to the internet.
With the person knowing the Passcode, they likely have complete access to all information. It is now up to you to secure any account that you use by changing the passwords and cancel any cards that may be in your Wallet app. Notify your cell provider as they may blacklist the phone, but this may also limit the ability for a cellular connection to the network for a remote erase. Assume that all data has been compromised.
And most importantly, contact the Police. The police report may be required for unauthorized access to financial accounts. Not sure how the thief knows your Passcode, but if you know the person then the police may have a better chance in finding your devices.
- How did the thief get your Passcode?
- How did you determine your device is in Airplane mode?