Is it possible to fix it if I install a custom firmware through itunes or quickpwn? Or what other options are there? Would geveysim work?

There is just one possibility if you have bootloader 5.8 you can still downgrade your baseband to version 2.28.00 by installing
3GFuzzyband downgrader available in Cydia(this app will also detect your bootloader version) and then install yellowsn0w from Cydia

You will have to jailbreak your Iphone first using QuickPWN.

Hope this helps

How can you accidentally update, just admit that you updated it cause you wanted to see whats new lol, you have to wait minutes til an iphone Restores how is that called accidentally.

iClarified - iPhone - How to Downgrade Your iPhone 3G Baseband Using Fuzzyband

FuzzyBand is the way to go if you were lucky enough to have the 5.08 bootloader preserved. It'll tell you if you've upgraded to BL5.09 "accidentally" as well

If you're on BL5.09 (with the 2.30.03 Modem Firmware/Baseband and FW 2.2.1), then you're going to have to go buy a Gevey 3G Plus 2 Sim for 2.2.1 (or any other 2.2.1 compatible turbosim) to get your phone working. Join the club and wait for the iPhone dev team to come up with the next yellowsn0w unlock.

G'luck!