Netcup wants to send me a letter in such a primitive way in 2024, which is really unimaginable.
That is the default way in Europe (especially Germany), it is also standard for (health) insurances and government processes.
Feels like stoneage sometimes, but it is a secure way. Sure it doesn't make a lot of sense for an international company, but I can't think of a good way to verify Chinese? customers.
They have tried changing the verification process via Stripe, but even then customers are unhappy.
Nevertheless:
Do you have a backup of your phone available? Do you have a backup of the 2FA credentials available?
Does your 2FA app syncs with the cloud somehow? (Google Authenticator does, but I'm not sure what app you are using in China?)
It's understandable that you are angry, however I don't think that accusations and blind venting in the Forums are helpful to your situation.