RF Combiner

also since it is reflecting energy back to the phone when the phone is transmitting, it will overheat the phone's RF power amp. high gain antennas are usually very narrow band devices, and the SWR (standing wave ratio) can climb beyond 3:1 as you get outside the bandwidth of the antenna. high SWR means energy reflected back to the transmitter, where it is wasted as heat. marginal overheating may not damage the phone right away, but could over time, degrade the output transistors. or it may trigger protection circuits that cut back on the transmitted power. there are also large signal losses receiving (google "antenna reciprocity") on an antenna that isn't matched to the frequency in use. these are ways the phone isn't performing as well as it would with the proper antenna.

Thank you very much Jed!
You taught me a lot.

That is the radiation pattern of the Antenna (attached), is it possible to see from there the SWR at below 2.1GHz?

no, it's not mentioned. gain and SWR of an antenna aren't related.

Thank you Jed.

Do you think there're significant reflections at 2.1GHz? (The pass band is 2.4GHz-2.7GHz)

As for the gain, I didn't manage to get from the datasheet what's the gain at 2.1GHz.
Only that there's 19dBi gain at 2.4GHz-2.7GHz.