is my iPhone 13 pro max original?

It should be an original phone then.

At this point, you have two options:

1. If you feel that the seller did not fully disclose the issue with this phone, you should return it for a full refund.
2. You can try having Apple repair the phone. Most likely, this will be at your cost. Depending on what they find, it could be quite expensive.

If you go with the latter approach, you can contact Apple Support directly by using the "Get Support" link in the upper right-hand side of this forum's web page.

Did you purchase it directly from Apple or an Apple Authorized Reseller? If so, it should be an original iPhone. If not, it could be possible that it is a fake.

What make you think that it is not an original iPhone model?

One method to check would be to enter the phone's serial number here:

• Check Your Service and Support Coverage - Apple Support

Did you buy it “new” from Apple (or an authorized reseller); and did you break the tamper-indicating seals yourself ???

Were all the components inside the box carefully wrapped in their own packaging.

Check your serial number on check coverage.apple.com