I just re-confirmed from my friend, his S5 is "rooted" and he installed "Xposed Framework" and then installed "LTE only mode for S5" from that framework. After these, it shows "LTE only" setting. But on LTE only, there is lots of signal dropping and weak signals at various places of Islamabad. So he had to set it back to auto. His network is Zong.
On auto setting, LTE automatically connects where there is high speed requirement and signals are strong enough (mostly in streets, upper floors or main roads).