Rarely, the iPad baseband can cause boot-loop issues on 3GS. Force DFU mode and restore to a stock IPSW (without the 6.15 baseband) – but note the baseband will remain 6.15.
Tools like iREB or RedSn0w are essential for bypassing the restore errors caused by this specific baseband. Custom Firmware With Baseband 6.15
Today, this baseband is mostly a relic for collectors or those reviving retro tech. If you're working with an old 3G/3GS today: Rarely, the iPad baseband can cause boot-loop issues on 3GS
Users
During this process, the screen would show a verbose string of code, ending with a progress bar uploading the Baseband data. It was a high-anxiety moment; a power failure or crash during this specific phase could "brick" the Baseband permanently, leaving the phone unable to make calls ever again. Custom Firmware With Baseband 6.15