Yes, I used that one, renamed to u-boot.ext and u-boot.sd (copy)
I have also tried many dtb but with the same result : reboot loop each 2 seconds (Vontar logo shows up for 1 or 2 seconds).
I have also tried doing the dd of an u-boot image just in case.
When I reboot from the android interface (update app), I have some access to kernel messages which looks like booting and showing some information like meson, but I can't find a problem in the log.
In the middle of it I remember I've seen that some file can't be found, but it kept continuing the boot.
@mradalbert said that he has the same box and has everything working fine.