From the datasheet, the only direction that seems to have notable deviation from the average is in the positive z direction, and slightly in the negative z (see the test setup image). Since Cellular antennas generally will be higher than your device it seems like having the z axis perpendicular to the ground is probably a safe bet.
Most simply, if the antenna is horizontal and not vertical it should be consistently similar/optimal for any rotation about z.
I also looked at external antennas, Alternative High-Gain 3G Electron Antennas
What I found was that the https://www.mouser.com/ProductDetail/960-TG.30.8113 antenna from Taoglas gave somewhere between a 3-5dB improvement in my Cellular.RSSI() values, with similar statistical distribution of max and mins. None of the others were significantly improved over the original (though the flexible antenova one is cool because it’s flexible).
However for us for now it seemed too small of a benefit given the extra certification requirements for a stronger antenna.
May I ask specifically why you need to improve reception? Does something not work as well as you would like, or is it just to be safe? If you don’t have a realtime need for immediate responsiveness you should be able to recover from some mild disconnection with a little caching of data.