Any particular model Sitecom wireless router and DLink access point?
Access points and wireless routers normally do not talk to each other (except using WDS or client mode if available). Is there some other client radio in the office that is going to talk to one or both of these wireless devices?
Where can you install this added antenna? 80 meters is at the outer limits of the range of wireless without added antennas. However, adding an antenna will do nothing if there are obstructions in the line of sight. A directional gain antenna will move much of the signal in one direction, but only at the expense of the same amount of signal borrowed from other directions. If you need indoor coverage on your routers, plus directional coverage to the office, this might require a more complex antenna arrangement. My guess(tm) is that at
80 meters, you'll need a directional antenna at both ends of the link. If you have line of sight, 80 meters is no problem. Without line of sight, it will be an "adventure". I can't offer any specific antenna recommendations without a clue as to the exact hardware and topology involved.