Q: How can I verify my iCloud Keychain? (there's a catch 22)

I have three devices on my account: an MacBook Air, an iPhone 6 and an iPad Mini 4.

Each requires activation for keychain.

I've tried to use the security code and it works but it sends a confirmation code to the phone number I've first used when I've created the account. Needless to say, that phone number is not active anymore.

I've changed the number in iMess. but this does little to help.

I've tried to follow online tutorials on how to change the number but I can't find any option to do so, neither in Apple ID, iCloud, MacOS or IOS. I've followed it to the letter. I can activate two step auth and such but I can't change the default number.

The only two options are "activate using another device" which does nothing (since all three devices need activation) and use a security code which sends a text to the old number.

I've followed the link "I don't have access to the number anymore" but it leads me to a page with the usual solutions. I'd go to an Apple Shop or call but I'm in Taiwan right now. Apart from the time difference and the cost to boot, for an international call, I don't know how they can help me verify my ID by phone.

So, what are your suggestions? Everything else works perfect. I have iCloud synced on all three devices. The only pending one is Keychain and it is getting very, very frustrating. I've searched through the options on IOS, on MacOS, even on Apple ID. I can't find a single way to change the phone number, even if every guide says that there should be one.

I've asked a similar question on Quora and the only answer I've got is that this is a known problem and the only way is to contact Apple ID. Can a mod help me with this or at least provide me with a way to solve this by email instead of phone? Business hours don't really overlap between US and Taiwan.

Thank you,

Razvan