H96 Max RK3528 - Cannot boot Armbian from TF/SD card

[Alexander Polko]

Hello,
I have H96 Max TV box with RK3528 chip (4GB RAM, 64GB eMMC, Android 13, build RZX.V01.20240924). Board: RK3528_DDR3_8X4_V12.
Problem: Box does not boot from TF card at all. No HDMI output, no DHCP lease, nothing.
What I have:

TF card slot on the box
1x USB 2.0 port, 1x USB 3.0 port
No serial/UART adapter (DEBUG header visible on PCB but not connected)
Android 13 still working on EMMC
ADB access with root

What I tried:

Built Armbian with BOARD=rk3528-tvbox BRANCH=legacy BUILD_MINIMAL=yes RELEASE=trixie
Extracted DTB from Android boot partition, added to image
Wrote image to TF card with dd
Verified RKNS header at sector 64 - correct
Tried replacing idbloader with one from our custom U-Boot build
Original U-Boot on EMMC appears to ignore TF card completely

Question: How to make original U-Boot boot from TF card? Or is there a way to install Armbian to EMMC safely while keeping ability to recover?
Device AIDA64 info:

Model: H96_Max_RK3528
Device: rk3528_box
Hardware: rk30board

Thank you!