I do not believe that Sprint has released it yet, but when it is available, this device will hook up to the internet and provide a cell signal like a mini-cell tower in your building:
Sprint AIRAVE
If you are inside a metal building, it is like being inside the shielding of a coax cable, so an external antenna might help a little but probably not very much. Also, the external antenna connector on the Titan is notoriously delicate, and allegedly the internal connection to it can easily break when plugging in an antenna, which will permanently disconnect the internal antenna also.
WARNING: boring technical info below
If you want to try the external antenna anyway, antennas are tuned by adjusting the length of the antenna; it should be an integral multiple of half of the desired wavelength. The wavelength is the speed of radio waves, which is the speed of light, divided by the frequency; so a frequency of 1900 MHz would have a wavelength of approximately 0.15789 meters, or approximately 6.216 inches. Therefore, an antenna of a length that is a multiple of 3.108 inches would provide the best signal.
See
Wavelength and Frequency, Univ. of Oregon