if I no longer have access to my old phone number?
Sign in to your Apple ID account page and edit / update your Apple ID details.
https://appleid.apple.com
> edit your trusted phone number.
You are sign in here . . . with your Apple ID & Password.
Your Apple ID is the personal account you use with all Apple products and services. It includes the email address and password you use to sign in, as well as the contact, payment, and security details that you use with Apple.
You can’t merge multiple Apple ID accounts.
What happens when you use the same Apple ID that you use to sign in here?
See -> If you can't connect or sign in to iCloud
If you can't connect or sign in to iCloud - Apple Support
Check your Apple ID via your email box, see this Apple article: If you forgot your Apple ID
If you forgot your Apple ID email address or phone number - Apple Support
OR see -> Having trouble signing in?
https://iforgot.apple.com/password/verify/appleid
See also -> Contact Apple for help with Apple ID account security
Contact Apple for help with Apple ID account security - Apple Support
You can also contact Apple support for help, -> Get Support (apple.com)
You have to show proof of ownership of your product, provide all documentation showing your purchase of the device.
If you do not have additional proof of ownership, contact the reseller. They may be able to provide the original purchase document.
Once Apple receive your proof of ownership, Apple will further review your request and provide an update.