Since updating to iOS 7.1, a number of users have complained of tethering issues — meaning that information doesn’t save when trying to entering custom APN settings into a device’s Cellular menu.
In the update, Apple has apparently disabled users from setting up Personal Hotspots on carriers who either aren’t official iPhone partners, or who don’t support the feature.
It seems that this is an attempt by Apple to crack down on unauthorized use of the Hotspot feature.
While understandable, it has caused a number of problems for users. The iPhone 4, 4s, 5, and 5s all appear to be affected by the issue, which has been noticed by users in Germany, India, Austria, Spain, Serbia, Thailand, Estonia, Hungary, Belgium, Italy, Brazil, and others.
If you’re having trouble with hotspots, check out the below thread on the Apple Support forum, which has now reached seventeen pages long
Source: Apple Support Community