You almost certainly cannot use the existing “large” antenna. Antennas must be resonant at their designed frequency, otherwise, they’re just a conductor and will not radiate. Resonant length is based upon the frequency. Lower frequencies require longer lengths to be resonant and higher frequencies are shorter lengths. A large antenna on your roof is probably an old TV antenna and resonant at lower UHF and VHF frequencies. This isn’t a good match at cellular frequencies. You can mount a specific antenna designed for your carrier’s bands on the same pole that the larger is on.
At cellular frequencies, the signal is line of site, meaning it will not travel over the horizon and will not usually refract off the atmosphere. Your donor antenna ( the outside one) needs to be as high as is practical/ possible to accommodate the line of site characteristic. I would recommend a directional antenna called a “yagi”. It should be mounted with as clear as possible path towards the nearest cell tower. The nearest tower that your carrier is on.