AT&T cannot give you this perk - in order to enable the tethering option on your device you have to be on a compatible plan, and no grandfathered unlimited plan is compatible (they all predate the introduction of tethering in iOS)

So you can either convert  to a metered plan that supports tethering, or not use tethering.

There is NO additional charge for tethering on the metered plan, you just prebuy more data at exactly the same cost as you would otherwise pay for that data (ie, +2GB + $20, or $10/GB) - you were gonna use that data anyway, right?