You cannot install the magisk.zip from the stock Android recovery, you need a custom recovery like TWRP. But if your device need to use the recovery image to install Magisk (due to no ramdisk in boot) you cannot install Magisk through recovery...so after hours nothing, i am sure something else is missing for this device, also while searching I found for the same allwinner brand a custom twrp for a TV box and it pass the signature verification, it does not work cause diffrence device, but when I try to load with the same method magisk zip it failed cause verification error, can we fix somehow the verification on magisk, searching for ways...
So far it sounds to me like you've managed to get a recovery image patched, but you have not yet managed to boot to a rooted system. I'm assuming you've read this:
When you do the recovery key combo to boot to a rooted system, does it boot up straight away or do you end up in recovery first? If everything is installed correctly it might just be down to a timing thing (as mentioned earlier) and you need to try different timings of releasing the buttons when the device vibrates with splash screen.
It could also be that the device simply is incompatible too, but let's hope it's a timing thing.