the “antenna” you are talking about is actually a crosspolar antenna. Physically, it contains 2 antennas (you can see that because there are 2 feeders ports at the bottom of the antenna)
Each port is internally connected to an independant “antenna”. Antenna “a” is polarized at 45°, antenna “b” is polarized at -45°.
The cross polar antennas receives the UL signal from the MS on its two antennas a and b. Both rx signals are different, one gets the 45° componant, the other the -45° componant.
The TRX receives those two signals and COMBINE them, in order to enhance the “quality” of the signal (interference cancellation, combining gain, etc)
in your case, Signal “a” can be called “main”, and signal “b” can be called “div” (stands for diversity). But those are just words. If you invert both cables main & div, it won’t impact anything.