Apple stopped selling the iP 11 2 years ago, and it is still supported. Best you can do is to submit a support form to start AppleCare + , and wait for Apple to reply to that.
If it's not an Apple authorized reseller, the warranty may have already expired as it typically starts at date of original consumer purchase regardless of whether the phone was ever opened/activated. Whether it still has the 12 month warranty will depend on how the phone was acquired by the "online reseller."
That would be true however most resellers sell phones which have previously been activated so you’ll have to enquire with them directly for a straight answer. The new/sealed claim is meaningless.