You have to hold power+bixby+volume up until you see the "splash screen" (when you boot there will be that yellow warning text after a few seconds that you have to skip via pressing the power button. You turn your phone off and hold power+bixby+volume up to turn it on (keep the buttons pressed). When that screen with the yellow warning text comes up you keep the buttons pressed for a bit (all of them including the power button)! That way the warning ("splash") screen will disappear right away and as soon as it does you release the buttons). Thats how you boot into magisk enabled rom. If you want to boot into recovery, you turn off your phone, hold power+bixby+volume up until the splash screen comes up and disappears again and release power+bixby (keep volume up pressed!). If you do exactly as i told you and it doesnt work im afraid you have to reflash everything. Ive made the experience that you have to do it right on the first time booting, let me know if it worked
Did you try any of the options stated by twisted61 ? I am actually in the same position but had not time to try it yet. However I share your concern. If your battery dies you have no chance to boot up your phone without having a computer near by to flash TWRP again, booting to TWRP, flashing ivans recovery and booting to it.If the battery dies and I keep getting the Galaxy s10 secured by knox screen. How can I bring it back (without a computer). I know I can flash recovery etc, but this is insane, no matter what button combo I could not get it out of that boot loop. Any ideas?