The Cel-Fi Pro consists of two units. There is the "Window Unit" and the "Coverage Unit".
The Window picks up the weak mobile signal from outside, either using it's own built in antenna, or from an external antenna connected to the unit by a cable.
The Window unit then sends this to the Coverage unit located further away and indoors. Normally the connection between the two units is by built in 5GHz WiFi, but there is an option to connect the units by cable.
Once the Coverage Unit receives the signal from the Window Unit it re-transmits this as a strong mobile signal - getting you from 1 bar (or even less at times) to 5 bars!
A mentioned the Cel-Fi Pro can work in two ways, using ir's own internal antennas, or being fed a signal from an external antenna.
The internal antennas of the Window unit are a high tech "Fractal" antenna and are really good at pulling in a weak signal. So if you have a spot in the building that get's "some" signal, simply locate the Window unit of the Cel-Fi Pro there.
The best place for the coverage unit is as far away as possible, whilst still maintaining a good link to the Window unit. This is easy to figure out as the display on either unit will tell you how good this link is.
It's amasing how often we find people convinced they will need an external antenna only to find the built in antennas are good enough to get a boost up to 5 bars.
Of course if there's no spot in the building with enough strength simply place the Window unit where you can bring a cable indoors from outside and connect up!
If using an external antenna there are 3 "options".
Where the reception is pretty good up on, say the roof, use an Omni (picks up all around) antenna. But if the signal is weak use a directional antenna like the LPDA to really focus in on the mobile signal available. In really weak signals a "Yagi" directional antenna may give better results.
To get the best "Boost" it is important that there is as much separation as possible between the antenna picking up the signal (The Window unit, Omni, LPDA or Yagi antenna) and the coverage unit. The coverage unit will push out the best signal when the Window Unit doesn't pick this up along with the external signal.
But as mentioned, most people are pleasantly surprised to find they don't need an external antenna in buildings with slight/marginal reception in one or two spots, and then find a convenient location for the coverage unit.
In some respects it's easier to place the coverage unit where an external antenna is used, so whilst needing an antenna and cabling the end result is often more flexible.
1 bar to 5 bars in 5 minutes - it really does happen!