I'm a profesional iphone user. I got same one on 12 Pro, and I just read your guy posts, but all didn't work for me, then I firgure out and worked for me now.
Following steps: Volume + and release
Volume - and release
Hold Side button for 6 seconds
Continue holding Side button and hold volume - for 6s more
Release Side button, and keep holding volume untill it get to DFU
GOOD LUCK !!!!
at503
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