Verizon and AT&T variants of the Galaxy S5 were found to be missing the Download Booster feature – which combines Wi-Fi and LTE connections to increase download speeds of files larger than 30MB – that Samsung has been touting ever since it announced its latest flagship, but it looks the option was simply hidden from sight. According to users on XDA, on the aforementioned devices, Download Booster can be accessed by creating a shortcut to it on the homescreen on a third-party launcher (like Nova or Apex), which will also create a new icon in the status bar representing the setting.
Unfortunately, whether Download Booster actually works this way is something that's unclear. According to one XDA user, enabling Download Booster gets him higher download speeds when using the SpeedTest app, though others say it doesn't do anything when you're actually downloading files. A single line of code in the system files can apparently be changed to enable Download Booster natively, but at the moment that isn't possible, since there is no root exploit available for either variant (though that should change very soon.)
If you want to test Download Booster on your AT&T or Verizon Galaxy S5, download a third-party launcher like Apex or Nova, long press on the homescreen, then choose Shortcuts » Activities » Settings » Download Booster to create a homescreen shortcut. Let us know if you see any noticeable difference in download speeds after you enable it.