Hi a few weeks ago I upgraded my iOS through iTunes for iPhone 3GS 8GB from 4.1 (Jailbroken & Unlocked) to 5.0.1 (Baseband 6.15.00), and then jailbreaking it with redsn0w + cydia -> as result - ultrasn0w unlock didn't work and my mobile provider was not descovered.

I have an iPhone 3GS with the last three numbers of the first 5 numbers higher than 135, so i have an iPhone 3GS made after the 28th week in 2011 and also it's a new boot rom. I've looked at many solutions, but my last solution appears to be flashing my baseband to the iPad's but my iPhone 3GS is not supported from that confirmed by this video (How To Unlock iPhone 3Gs 5.0.1 for Tmobile & Fix No Service Error - 5.16.05 to 6.15.00 - Ultrasn0w - YouTube).

I tried restoring, and jailbreaking it again, and still I receive the 'No Service' message. I am aware that flashing to the iPad baseband means i can no longer get a stock IPSW or downgrade.

So now i'm currently on 4.1 Jailbroken but with 'No service', So I decided i'm going to purchase an AT&T Sim card from Amazon. So I see that everyone goes to ultrasn0w to unlock their iPhone's to any carrier, i had this luxury as well, I want to know, if i buy a never activated, AT&T Sim card and restore my iPhone 3GS on 4.1 and activate it with that, will i be able to then switch it/swap it out with my carrier here in The Bahamas' SIM card or any carrier for that matter? What is the process of doing this and also having a 4.1 jailbreak as well?
Thanks in advance.

You have (permanently) bricked the baseband. Attempting to install the 06.15.00 baseband on a phone with a S/N that shows it was made newer than the 35th week of 2011 has permanently destroyed the baseband of this phone and it will no longer work even on the carrier it was intended to work on, and there is no way to fix what you have done via software. You will need repair either by replacing the baseband chip specifically or the entire logic board and comms board assembly.

Sorry to be the bearer of bad news but we have stickied posts in our forums about this specifically telling people NOT to attempt the 06.15.00 baseband update on a phone where the 3rd, 4th, and 5th numbers of the S/N are 135 or higher.

Originally Posted by n1ckn4m3

You have (permanently) bricked the baseband. Attempting to install the 06.15.00 baseband on a phone with a S/N that shows it was made newer than the 35th week of 2011 has permanently destroyed the baseband of this phone and it will no longer work even on the carrier it was intended to work on, and there is no way to fix what you have done via software. You will need repair either by replacing the baseband chip specifically or the entire logic board and comms board assembly.

Sorry to be the bearer of bad news but we have stickied posts in our forums about this specifically telling people NOT to attempt the 06.15.00 baseband update on a phone where the 3rd, 4th, and 5th numbers of the S/N are 135 or higher.

I think i'll take your advice, i have an iPhone 3GS 8GB... Is this what i need to purchase to solve my issue? And is the comms assembly included in the logic board? $130 (Incl. iPod removal tool) Is way more affordable than $350 for a new one.
iPhone 3GS 8GB Logic Board - Working

If so, what would be the method of unlocking my phone to my carrier here in the Bahamas? The regular jailbreaking procedure i assume?

Doing a quick once-over, it looks like the piece you'd need. I searched for a few minutes to see if I could find just the Toshiba baseband chip that you'd need to replace, and while I found a place in Beijing that would do it, I didn't find a place to actually buy the Toshiba baseband. Was hoping it would be a cheaper repair that way for you.

In any case, that looks like the piece, the full logic board assembly will contain the baseband as well as SIM slot, so it should be an all-in-one replacement. My only concern is that the product is listed as 'used', and though it's also listed in working condition, I'd spend a little time seeing if there was a new one to buy. It's got a warranty from the provider though, so hopefully it won't be a problem.

The bad news is that once you do this, you still won't be able to unlock the phone. Short of the Gevey SIM / Turbo SIM / SIM Interposer solution, there is no way to unlock the new 8GB 3GS yet -- it uses a Toshiba baseband that isn't susceptible to ultrasn0w, and that can't be upgraded to 06.15.00 (as you've experienced). The ONLY way is the interposers at this point, and I don't believe those require a jailbreak -- just the device.

Do you think it would be more feasible to buy a new iPhone 3GS as opposed to this method? Because if it cannot unlock, there's not use.

The problem is that buying a new 3GS, you're going to get a new one with the new baseband chip in most cases. It's a gamble either way. Since none of the currently produced 3GS models can be unlocked, if you absolutely must have an unlock, you'll need to use one of the Gevey SIM / Turbo SIM. I really don't think you'd get one old enough to be able to use the 06.15.00 baseband.

That said, you can find a LOT of the devices on the 06.15.00 baseband on e-bay, and they're fairly cheap.

Unlocking it is the only way i'd be able to use it here in The Bahamas. Thanks fro the help, I guess i'll just buy a new iPhone 3GS and use my discretion when upgrading/jailbreaking.

Hmm, i don't quite understand your eBay reference, is this what you mean?
Digital Baseband Processor CPU 337S3754 Replacement Parts Repair for iPhone 3GS | eBay

Buying a new baseband?