Which country is your iPhone 11 Pro Max from? Maybe it’s from one of China’s territories (China, HK or Macau) – so they have physical two nano sim card instead of eSIM?
I’m on XR got a 1GB plan, used it up, deleted and tried to use the code again. Tried different accounts, all didn’t work. I’m pretty sure they can detect the same device so they won’t let you use it twice on the same device. Good tutorial though