NB - I am in NO WAY responsible for your phone conking out, frying, dying, hanging, bootloops, or any other incident, by running below mentioned procedure.
That said, do thank me if it works for you!
If you have Xposed installed, then search for an install it from there.
If not, then use:
http://dl-xda.xposed.info/modules/com.smartmadsoft.xposed.s5.lteonlymode_v1_3ec9a0_0.apk
to download, and run (phone should be rooted).
This mod adds "LTE only" option in Network mode (in system settings). Unlike using secret codes (which is less convenient), this is persists during boot.
Intended only for Samsung Galaxy S5 SM-G900F and possibly other S5 GSM/LTE models.
General info:
LTE is 4G technology and HSPA+ (or 4G) is 3G technology end of story. That doesn't mean HSPA is bad or poor service, its fantastic service and has great speeds, but its not 4G.