iplay 7t (sc9832e processor) root / unlock bootloader suggestions

Recently purchased an iplay 7t after reading the xda review. This is replacing an LG v400 tablet that I had rooted. I updated the iplay to build T701_V1.20_20191112, enabled developer options, enabled unlock bootloader in developer options, found the corresponding firmware pac, installed magisk and used it to patch boot.img. So far so good.

I entered fastboot, then I attempted to flash the modified boot.img and was told:

target didn’t report max-download-size
sending ‘boot’ (18584 KB)…
OKAY [ 0.593s]
writing ‘boot’…
FAILED (remote: Flashing Lock Flag is locked. Please unlock it first!)
finished. total time: 0.608s

I tried various options to unlock the bootloader:

> fastboot getvar unlocked
unlocked:
finished. total time: -0.000s

> fastboot oem unlock
…
FAILED (remote: unknown cmd.)
finished. total time: -0.000s

> fastboot oem unlock-go
…
FAILED (remote: unknown cmd.)
finished. total time: 0.002s

> fastboot flashing get_unlock_ability
…
FAILED (remote: Not implement.)
finished. total time: -0.000s

> fastboot flashing unlock
…
FAILED (remote: Not implemet.)
finished. total time: -0.000s

> fastboot flashing unlock_critical
…
FAILED (remote: Not implement.)
finished. total time: 0.016s

> fastboot flashing unlock_bootloader
fastboot: usage: unknown ‘flashing’ command unlock_bootloader

> fastboot flashing unlock_bootloader_nonce
fastboot: usage: unknown ‘flashing’ command unlock_bootloader_nonce

Okay … fine. I fired up SPD Research tool and attempted to use it to flash the modified boot.img. It transfers the image and then times out.

As a sanity check I used SPD Research tool to flash the original boot.img and that worked fine.

How do I unlock the bootloader on this unit?