From what I read apple stopped signing 4.3.5 like just a few days ago, so you are unfortunately out of luck. Restoring back 4.3.5 would have downgraded your modem firmware to 4.10.1 (I think). Anyway, never upgrade without preserving your baseband if you depend on the unlock.
The only thing you can do is ask the original provider for an unlock, wait for a new hack, or sell your iphone...
IPhone4 (Sim Locked) showin no network 4.11.08 baseband
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By mistake i upgraded to ios5 with the 4.11.08 modem firmware(baseband). Now Gevey Sim unlock does not work. is there any way to undo this upgrade mainly the modem firmware.
Gevey Sim: www.gevey.com