iCloud reset password not working

If the old Apple ID is signed in the previous device ( iPhone ) you are not able to reset Apple ID password for secondary authentication account convert the account into 2FA Two-factor authentication for Apple ID - Apple Support

Get a verification code and sign in with two-factor authentication - Apple Support

On new iPhone set up Set up your iPhone or iPad - Apple Support

You have to use the old Apple ID and password on the new iPhone and sign in , on the same network so that both will sync .

Note : the previous old Apple ID should be compulsory signed in the device ( it could be say example1@gmail.com if not it will be a simple mail address as if you use this link https://iforgot.apple.com/appleid#!&section=appleid it will not work when all credentials of the account are not remembered .

The Apple ID is not signed in the iPhone or device being used , the advisor will verify the account by asking few questions ( the account credentials ) when you contact them on phone Contact Apple for support and service - Apple Support 

The serial number of the device , Apple ID address should be known to you , don't pretend to be another account holder , every details are in the records .

( a ) If you lose answers for Apple ID provide credit card details that was added in the account .

( b ) if you have lost Apple ID password provide invoice copy of iPhone to apple support advisors .

( c ) Contact apple support , they will send a link on mail account , click on link after 24 hours to reset apple id password , however the link is valid for additional 3 hours .

( d ) Support pin is used by apple support advisors to verify the account when you know apple id password Generate a temporary Support PIN - Apple Support