He enters through the side door or the main door.
Here's why:
* "Through" is the preposition needed to show the way of entry.
* "The" is required before both "side door" and "main door" for clarity.
* "Or" connects the two options for entry.
The original sentence is grammatically incorrect because it lacks the preposition "through" and uses "door" without an article before it, which is ambiguous.