How to downgrade iPhone 4S from iOS 9.0.1 to iOS 7.1.2? [duplicate]

(Please read the whole question before flaming me)

Today I updated my iPhone 4S (Model A1387) to iOS 9.0.1 from 8.4. Since the Wifi doesn't work with either of these versions I decided to downgrade the software to iOS 7.1.2.

After I downloaded myself a IPSW-File for the iPhone 4S and tried to [Shift]+Update my phone in iTunes I received the error code 3194. After a quick web search I booted the iPhone into recovery mode and tried it again. After that I tried adapting my hosts file in Windows as showed on apples support page. This also didn't work. I also tried downloading the I P S W-File from different providers and also tried to use a third party tool called "The Firmware Umbrella". Then I found out, that recovery mode is not enough and I have to go in DFU mode.

Since the black screen is an indicator of the DFU mode I was sure that it will work this time, since the DFU mode seems to be some kind of ultimate firmware override mode, but after hours of effort I'm still stuck with iOS 9 and nothing has worked out to bring me back to iOS 7.1.2.

So as the title states: How can I downgrade my iPhone 4S to iOS 7.1.2?
(As I am going to jailbreak this device afterwards, you can also show a direct way to load a jailbreaked firmware)

Solution 1:

You cannot downgrade to anything less than 8.4.1 now. Apple signs iOS software, they currently only sign 8.4.1-9.0.1. This means that you can't downgrade to 7.1.2.

Solution 2:

Only way you can do this is if you have already saved SHSH blobs from the time when iOS 7.1.2 was current. You can no longer get them, nor can you use someone else's (as they are unique to the device). So unless you already have these blobs you cannot downgrade

More info

Apple signs iOS software and whenever you upgrade or restore, iTunes will first ask Apple servers if the upgrade is valid. At the moment (27th September 2015) only 8.4.1 and above are being signed. This means the only way to go lower is to sign the restore file (IPSW) yourself and this can only be find using valid SHSH blobs