The two-factor code should go to all trusted devices, including your iPhone 6s if you still have it. If not, the following has instructions on how to have the code sent to a trusted phone number (which could be an Android or even a landline) if you set one of those up: Two-factor authentication for Apple ID - Apple Support
If you have no trusted devices and no trusted phone numbers, then see the following from the same link:
If you can’t sign in, reset your password, or receive verification codes, you can request account recovery to regain access to your account. Account recovery is an automatic process designed to get you back in to your account as quickly as possible while denying access to anyone who might be pretending to be you. It might take a few days—or longer—depending on what specific account information you can provide to verify your identity.
Losing phone number of setting up of Two factor Authentication