My girlfriends phone broke this week and I gave her my spare (old) Samsung Galaxy S4, which I had replaced due to glitches (like not seeing the SIM card regularly). But it worked most of the time and a simple reboot would fix the issue. At the end of the next day she told me that her mobile internet was not working and I was surprised. I did not modify anything, so the Telfort configuration must have come from the network. On their website you are able to find the correct settings, but you have to follow a terribly inefficient, phone specific wizard and are guided step-by-step (e.g. user friendly) to correctly configure the settings. The settings basically boil down to:
APN: internet MCC: 204 MNC: 12 Accesspoint type: default,supl
Then I simply saved and enabled this and all was well again.
For those who are interested, a proxy server (with port) was configured and encryption was enabled.