AMD Radeon 570 - monitor not detected - resolution problems

I guess this graphics card requires non-free software, which linux-libre refuses to load. Since I want to avoid that non-free software long term, I am contributing to a project that develops a freedom respecting 3D one. But that will take a long time until we have that is usable in the free world, reverse engineering might help. In the case of NVIDIA, this won't help since the non-free firmware is signed. Older AMD ones might work with linux-libre, a different kernel version also might help.

Nothing newer than a R9 270 and R7 370 has worked for me on the libre kernel. My RX 460, RX 580, and RX 590 all have the same issue as you. It's because they require closed source firmware which the libre kernel won't load. My testing has been with the 6.9 kernel, I'll be testing the 6.10 kernel in the next few days and will let you know if I have any luck. Trisquel 11 ships with the 5.15 kernel so it won't work out of the box just yet regardless.

"but I am wondering why it happens, since other OS like mint and deb are working at the right resolution"

Mint allows closed source Firmware. Trisquel is one of a few distros that use the Linux libre kernel which allows no closed source firmware from the kernel. Better for privacy, security, and user freedom but worse for hardware support. That's at the fault of the hardware manufacturers for making their products use closed source firmware and not making it FOSS in my opinion.

So I tried libre kernel 6.10 on my R9 270 and RX 590. Pretty much no change from kernel 6.9. The R9 270 was still suitable for basic usage and could adjust resolutions and refresh rates just fine but 3D gaming was a slideshow. The RX 590 could boot into Trisquel but couldn't recognize the monitor, change resolutions, etc. Before kernel 6.8 though pretty much any AMD card either wouldn't boot into Trisquel at all (like my 590) or couldn't change settings so progress has been made in that there are some options now vs having no AMD options. Hopefully as more work is done more AMD GPUs can become usable on the libre kernel. It would help if AMD would make their firmware FOSS but I know that will never happen.

thanks for the comments.
I was sure AMD had free drivers for all the video cards, but this, as I learned here is not the case.

I am still confused about the right terms to use, thank you for the clarification.
Anyway, I'll change the video card with a supported one.
edit:
I hate them, why is not illegal non free software?