I have the same problem. i followed the intructions:
- downloaded and installed Magisk Manager on the phone
- downloaded the July image file from the link (my phone displays July security patch as the present version)
- rebooted with adb reboot-bootloader
- rebooted with fastboot boot 5_July-2020-patched(by-techmesto.com).img
- Terminal diplay says downloading and booting the boot.img as 'OKAY'
- the phone reboots, but not into magisk, but into the normal Nokia/Android 10
Have I missed something? I have a Nokia 7.2 / TA-1195 with Android 10 and security patch July 1. 2020. Newer security patches are available but fail to install. The bootloader was unlocked by you 2 weeks ago and I'm trying to install a AOSP ROM ever since. I got stuck in bootloops a few times but so far no success in installing Magisk or TWRP. Have you got any advice?