Probably need to EDL flash the last released rollback file then do some formats from recovery before going back further but I'm no authority on this. Let someone experienced with modding A7 post with certainty.
No need for edl tool. Get Firmware rollback from ZTE Axon 7 support site https://www.ztedevices.com/en/suppor...4A3D2BFCF6AC7E There you can get a firmware Android 8.0 retreat to Android 7.1 that let you downgrade back to Nougat, it's about 2.78 Gb or thereabouts.
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