Hi there, there are lots of posts out there of people asking for a solution to this problem so I wanted to share the the way I solved it (completely by accident).

• If you're using rom version A202FXXU3CUJ1, download the boot.tar file that I patched, ready to flash, from here.
  

1. If the link is not available or you want to patch it yourself, extract file boot.img.lz4 from APxxxx...tar.md5 file.
2. Move boot.img.lz4 to your phone.
3. Install ZArchiver from PlayStore.
4. Open ZArchiver, navigate to the location of boot.img.lz4 and extract it in order to get boot.img.
5. Open Magisk, select Install and open and patch boot.img.
6. Open ZArchiver again, navigate to the patched file, long press on it and select "Compress" and extension ".tar". Generate the file.
7. Copy the compressed file back to the pc.

• Flash it with Odin in the AP tab
• I didn't have to wipe data or anything else, next time the phone started Magisk showed as installed and no data was lost. It's not necessary to boot the phone on recovery in order to get root access either, it remains rooted even after turning it on with only the lock key.