Because it took me about 10 minutes to Google it, I’ll recap it here;
If tethering is unavailable on ones iPhone, and you have to helpdesk him/her out of it, go to:
Settings > General > Network > Cellular Data Network
And set “Cellular data” to:
APN: web.be username: web password: web
And reboot the device… Tethering should/will be available.