There might be a very jerry-rigged way to do it via iFile (or SSH) and .plist editing, but it's both risky and messy...
Open up iFile or your SSH/SFTP program of choice, and navigate to /var/mobile/Library/Carrier Bundle.bundle/carrier.plist.
Edit that .plist file, editing the field "Show3GSwitchOnCat10HW" from "Off" or "false" to "On" or "true." If that entry does not exist in your file, add it like so at the end of the file:
right before the final </plist> tag.
You may need to reset your network settings (possibly several times) for this change to take effect. Please be aware that this is a very messy, quite risky, and untested method. You may need to restore your device, losing your jailbreak, if this does not work as well as anticipated. You attempt this procedure at your own risk.