Here is the difference between the description of the guard tower upgrade and the keep upgrade -
Upgrade to Guard Tower:
Upgrades your Watch Towers and lets you build Guard Towers, which are stronger and have more attack strength.
Upgrade to Keep:
Upgrades Guard Towers and lets you build Keeps, which are stronger and have more attack strength, range, and armor.
All that being said, there is no current range increase as described in the description.
Whether it be +1 or more is besides the point, it’s simply missing.