Q: mobile hotspot

Settings > Cellular > Personal Hotspot. Note that it must be enabled on your cellular data plan. More info: http://support.apple.com/kb/HT4517

Since the carrier verified you do have tethering on your plan, try restarting the phone. If that doesn't work, try resetting (hold the Sleep/Wake button at the top and the Home button at the bottom front at the same time until Apple logo appears). Don't know if either will help but it's worth a try if you haven't already. Then look under Settings > Cellular for Personal Hotspot again. On my phone it is both under Settings > Cellular and there also directly under Settings > Personal Hotspot.

http://support.apple.com/kb/TS2756

I'm out of ideas other than to contact Apple support.