This isn't going to work.
For microwave devices, signal loss in the cable is enormous, so unless your antenna is within several meters of the device, there won't be any signal for it to emit.
Your best bet would be to install powerful antennas in the basement, next to the device or to get a repeater. There are some interesting directional antennas which you could use. If the device is going to be in the basement, you could just point the antenna to emit up.
Also, make sure that the antenna supports frequencies you are going to use. This could be a problem if your router has 5GHz mode.
An antenna cannot by itself add more power to the sent or received signal. It can, however, focus the power from the signal, sending more of the power into some directions and less of the power from the signal into other directions. The same applies to the reception: an antenna will be more sensitive to signals from some directions and less sensitive to signals from other directions. Thus, for some directions (hopefully the ones you are interested in) the signal (and the sensitivity) will get stronger, and for other directions (hopefully ones you are not interested in) the signal (and the sensitivity) will get weaker.
This is measured as a number, the antenna gain. The bigger this number, the more the antenna focuses the signal in one direction. However, if the antenna is less efficient (for instance, if it loses half of the signal as heat), this number will also decrease.
A theoretical antenna which spreads the signal uniformly in all directions (an isotropic antenna), with 100% efficiency, is the reference usually used for this number. However, such an antenna does not exist. The lowest you can get (assuming 100% efficiency), according to Wikipedia, is a gain of 1.76dBi.
According to the page you linked, the router comes with antennas with a gain of 2dBi. Looking at the picture, it seems they are some kind of omnidirectional antenna, as is common for domestic routers. These antennas, when placed on a vertical position (like in that picture), focus more signal on the horizontal direction (on the same floor), and less signal towards the floor and the ceiling (which is what you usually wants).
So, to answer your question, if you replace your antennas with "better" ones (with higher gain), the result would probably be:
- It'll magically provide a better and/or further signal in some directions
- It'll magically provide a worse and/or nearer signal in some directions
So, if you for instance bought a set of 5dBi omnidirectional antennas, and put them in the correct position, the signal would be "better" on the horizontal plane (probably what you want) and "worse" above and below the router (also probably what you want). As you go to much higher gains, the antennas start getting very directional, which might not be what you want.
As an aside: sometimes having a "worse" signal is actually what you want. The more powerful the emitted signal and the greater sensitivity, the more you overlap with other access points and stations. Remember that wireless is a shared medium, so you share the available bandwidth with all the access points and stations you overlap with. Reducing the radius of overlap can sometimes be helpful.
Best Answer
If you're doing a long-distance point-to-point wireless link over 802.11a, b, or g, then you just want the "main" (not "aux") antenna connector to have the high-gain antenna hooked up to it, pointed at the high-gain antenna on the other end of the link. Lock your equipment to using just the main antenna.
Update: Oh, you're using omnis. Read on.
If you're using omni-directional antennas, then shame on your antenna vendor for claiming that they're high-gain. I don't usually consider omni-directional antennas to be high-gain, despite how some vendors choose to advertise them. +3-9 dBi like most omnis get isn't a lot of gain compared to a directional antenna like a typical yagi.
Anyway, in the case of omnis, realize that they get higher gain in a 2-dimensional disc by sacrificing performance in the directions above and below the disc. Yes, it can be kind of confusing that "omnidirectional" doesn't really mean all directions, it means all directions in a 2D plane. An antenna that works equally well in all directions in a 3D sphere is called an "isotropic" antenna. In fact, directional antenna gain is measured in terms of how much more of the signal goes in the primary intended direction as compared to a theoretical perfectly isotropic (spherical coverage) antenna. That's why antenna gain is reported in dBi. That's deciBels referenced to an isotropic antenna. Think of antenna coverage patterns like a water balloon. A typical water balloon is basically spherical. But of you hold it and squeeze it in one direction, it extends in that direction, but contracts on the sides where you're applying pressure. So a directional antenna "gains" performance in one direction by sacrificing performance in other directions. An "omnidirectional" antenna gains performance in a 2D disc by squishing the top and bottom of the sphere. Think of a water balloon squished between two books.
If this is a single-story building or an outdoor application over flat ground, then adding a second pair of omnis isn't going to help much. But if you're dealing with a multi-story building or interesting terrain, then it could help to add another pair of omnis oriented perpendicularly to the omnis you're already using. That way you get more coverage in, say, the "vertical" direction, instead of just the "horizontal" direction.