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Client of T-mobile having an iPhone 4 & iOS 7.1.2 - problem with connecting to the Poland's network [3]
I never had an iPhone nor I ever will.
The problem you are having is as follows, during update your APN was erased.
Newer devices might not to be up to speed with the mobile provider(s) or vice versa,
plus devices like the iPhone only have a limited number of these carriers built in (usually the carriers which have an official relationship with Apple)
For example T-Mobile in US just recently started selling iPhone.. so in older iPhone devices T-Mobile network wasn't listed in APN.
APN=Access Point Name
BTW, is your iPhone jailbroken?
APN principle applies not only to iPhone but Android and other OS's
From what I understand T-Mobile in Poland is not that long---I judge this from conversation with my friend from Poland who never heard of T-Mobile before, about 2 years ago.
First get in touch with T-Mobile rep in Poland and see what he/she has to say.
Solution # 1 revert to previous OS version (i think there is a time limit that you can do so)
How to link : daily.bhaskar.com/article/GAD-unhappy-with-ios-7-here-is-how-to-downgrade-to-ios6-4380650-NOR.html
Solution #2a go to PDA/Smarthphone Forums and dig for answers. Do not settle for anything less then this following Forums. I give you a link to the similar thread for starters.
Do not pay attention about different devices, principle for APN is the same.. just Apple is messing things up.
link: forum.xda-developers.com/showthread.php?t=2586058
Solution #2b go to iPhone Forum, regarded one of the top ones for Apple.
link: forums.macrumors.com/showthread.php?t=1642124&highlight=after+upgrade+lost+apn
Last but not least, use common sense, if not sure what you are doing, don't do it.
I take no responsibility whatsoever if you brick your phone. XDA Forum is for developers.
Good Luck.