You must have at least 2 spare cables from old smartphones etc. Just cut 2 of them, keep the USB-A parts and connect the 4 thinner cables color to color. Insulate the connections with electrical tape. Do not make it longer than 15-20cm.
I am sorry to inform you that you are not the only one with such problems. There is a number of other Vontar X3 users with such problems even from 1st use !! And probably the UART would offer you nothing new, as some others already used it (no solution from its use). Probably the Emmc is still your best bet (heating and even slightly pushing it at the time of UBT "burning" ?).
But first of all make a proper cable !!! and try again. Anyway such a USB-A/USB-A cable is a must for everybody (even for your next box

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