I believe 1800 is the default max without upgrades because thats the sight radius limit.
It may be possible to attack beyond the sight radius although the tower won't be viewing it's target in fog of war. The sight radius max is 1800 (different sight radii for day/night) while the acquisition range and actual range can go beyond that. You need to look at the acquisition range first and then the sight radius and finally the actual range itself.
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