SteeMan

Have you tried all of the gxm dtb files? Per the instructions (https://forum.armbian.com/topic/33676-installation-instructions-for-tv-boxes-with-amlogic-cpus)

In the world of TV boxes "identical looking" means nothing, as manufacturers build each batch with the lowest cost components available at the time. There are many cases where identical looking boxes even contain different cpus ( ie amlogic vs rockchip). My recommendation would be first to verify the markings on the boards (there is usually some sort of board identifying text on the board)to really see if they are the same under the hood.

You aren't using Armbian. The balbes150 builds were a fork of Armbian and know one here would know what sources they were built with. So this question is really off topic for Armbian.

Note that I believe Armbian will reset the max CPU frequency on boot. There is code in /usr/lib/armbian/armbian-hardware-optimization that uses the values defined in /etc/default/cpufrequtils to initialize the governor, min and max frequencies at boot time. So even though cpufrequtils is no longer an installed set of tools, Armbian is still doing some of the same work based on its old configuration files. So assuming this works as it used to, all you should need to do is set the max speed you want in /etc/default/cpufrequtils

There aren't any Armbian builds for 5.15 is what I said. And it would be a lot of work to make that happen. Why do you want 5.15? How do you know that would work, when 6.12 doesn't?

There are no Armbian builds that old, in fact Amlogic TV boxes weren't incorporated into Armbian until later Linux kernels. What you must have been using would most likely have been a fork of Armbian. What you are asking for isnt possible in the way you are asking. You essentially want Armbian ported to the 5.15 Linux kernels, which is a huge project. You would be better served but spending a month or so figuring out why current kernels don't work for you and fixing that.

Moved to proper forum

Moved to off-topic as this isn't Armbian related

What you now need to capture is the initial boot loader messages, what you are showing here look to me to be messages from a booted android linux kernel

I don't know what pins you are connecting to on the board as your video doesn't show that. But if I had to guess the three pads labeled gnd,rx,tx just above what you are connecting to is what you want to be connecting to.

poweroff doesn't work on any of my boxes either. Does reboot work correctly for you? It does on my boxes, and I usually issue a reboot and then time a power disconnect when the drives have spun down and before they restart.

Only a guess, but the first thing I would look at is how this is connected to your t box as it seems to not be connecting correctly. Maybe some pictures of your connection points?

Why are you expecting this to work?

Moved to amlogic TV ox forum

You would just burn your downloaded image file to any USB storage, just like you would to an SD card (i.e. plug the USB storage into another device and burn the image to the storage)