hi woetjes to clear that problem just fill your internet tether with the same entry you have given in your apn settings
general - cellular - cellular data network - scroll down to internet tethering fill the APN entry space with the same you have filled for your cellular data apn.
now just turn on and off the cellular data by just toggling . the problem will be solved.
first reset ur ineternet cettings and then only yu do the above .