If you find the Internet tethering option is missing after blackra1n jailbreak, try to follow these procedures to bring it back.
1. First, make sure you’re using blackra1n to jailbreak your iPhone
2. Next, on your iPhone, launch Safari and point it to http://help.benm.at.
3. Scroll down and tap on “Mobileconfigs”.
4. Select your country and then your existing carrier. Say, you’re using AT&T, select “U.S.A.” and “AT&T”.
5. Tap “Install” and then select “Install Now” to confirm the installation of carrier profile.
6. After installation, restart your iPhone (i.e. Power it off and then start it up again)
7. If everything is fine, go to “Settings” -> “General” -> “Network”. The Internet Tethering option should appear.
When you need to use Internet Tethering, switch the option to ON to enable it.