It's "pre order" and I don't think it's a thing you can really get today. Meanwhile there are small PCs that you can buy today which will work well, like Zotac Zbox CI329 Nano, or HP Elitedesk...
And of course, there's always RPi4 with UE300, which including case and power supply and SD Card, is out of the box like $120 or less.
That's incredibly weird. I've got an RT3200 too (or, rather, two of them), and I can easily manage 650-800Mbps over WiFi, around 900Mbps if speedtest runs on router, and 940-960Mbps with local iperf3 over ethernet.
And to address the ZimaBoard - it's a promising thing, but not released yet, and compared to prices of much more powerful x86 based boards, it's expensive. You'd probably be better off picking one of the recommended x86 SBCs.
Please use the commands I've pointed out to you in the RT3200 thread. This screenshot is useless as it doesn't show nearly enough information.
Run uci show network and uci show wireless (make sure to remove sensitive information, like SSID, MAC addresses and WiFi PSK), and paste the results here.
I see you're running a VPN. Is that active when you're testing? That might explain the speed drop. Otherwise I don't see anything that would be too different to my config.
How far is the router from the device running the speed test? I presume it's through WiFi. Maybe the other router has stronger signal, or the antennas are arranged differently, providing better coverage? Have you tried running a speedtest from the router itself?
Looks like your devices are using 2.4GHz WiFi instead of 5GHz. Try setting the Operating frequency - Width of that adapter from 20MHz to 40MHz, and set channel to auto. That should improve things.
You should also check why your devices are using 2.4GHz instead of 5GHz.
I meant 2.4GHz since that was the context. 40MHz on 2.4, 80MHz on 5GHz (it would also support 160MHz but in my experience it's been incredibly flaky).
So you have at least one main wall (or in this case, floor) between the router and the device. That would also explain the signal drop (the RT3200's antennas/signal aren't the strongest).
Well that block of "ground" (which I presume is reinforced concrete, an amazing way of blocking signals) is the issue then. Try a speedtest with a device closer to the router, or even better, through wired connection.
AX or AC, doesn't matter - though AX is generally better for "forwards compatibility" (e.g. if you get a new phone/laptop with WiFi 6, you don't need to tinker things).
The country setting is used to determine available channels, set it to France since the "driver default" will restrict the use of channels 12-13 (which might be under-utilised in your area, meaning less interference, better signal).