Then you're doing the procedure wrong. You should be able to place your device into DFU even if you're boot looping, because DFU comes before the bootrom/LLB is started.
Turn your device off while unplugged. Then plug it into your computer immediately home the power and home buttons together for 10 seconds, you should be at a black screen, then release the power but keeping holding the home. Timing is everything when your device is boot looping
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