iOS 7.1 Personal Hotspot APN settings won't save on iPhone 4

Hello,

I am facing a problem, which would be great to resolve. Since I updated to iOS 7.1 I can't set APN setttings for Personal Hotspot in my iPhone. If I go to Settings -> Cellular -> Cellular Data Settings, and I set the APNs(for data+mms) and i go back to save the freshly set APN settings then it saves, but if I touch any fields in the "Personal Hotspot" section (even focusing) and I go back the same way any changed value won't save (e.g. I also change data APN). In the menu "Cellular" there is an option for personal tethering where it just keeps loading and after a ehile it becomes an available option, but if I click on it it displays an error message like: "...contact: Carrier". So I tried everything, I reset my network settings, did 3 full restore(via iTunes, in recovery mode and DFU too). Then finally I downgraded to iOS 6.1.2 and guess what, it started working.. Then I thought it repaired some settings bug which burned in my device and I restored 7.1 in DFU. Now it is not working again. I need the help of community since I need hotspot tethering so hard everyday for my job. Thanks in advance!

Péter Józsa