I am just experimenting at the moment with my Raspberry Pi 4 Model B with 21.02.3 installed, I just created image straight from Openwrt repository.
No USB dongle, just with the built-in wifi, I can configure it as a 5GHz access point with ease - so a typical scenario where the LAN port is connected to upstream gateway and wifi (5GHz) configured as access point. Other devices can connect to this wifi and are connected to the internet.
The problem comes when I try and change the band from 5 to 2.4GHz (using the luci interface). Then 'ifconfig' doesn't show the wifi anymore. 'iw wlan0 info' shows the channel to remain in the 5GHz band. It just seems the me the device refuses to switch to the 2.4GHz band.
Already checked the wiki page of Raspberry Pi 4. The section on updating wifi firmware doesn't seem to apply anymore, as 21.02.3 already contains the latest firmware.
Any hint appreciated. I'd like to use 2.4GHz as the AP frequency, without any extra USB wifi dongle attached to it.
At the moment, I can't answer the question about your issues with the Pi 4 2.4GHz radio, but I will offer this: it's not worth spending much time on this if you care about wifi performance in general. The Pi has very poor wifi that is primarily designed to work as a client only. It has a very low end 1x1 radio system that can be used as an AP, but it is not able to efficiently handle multiple client devices. And because the antenna is very small, it isn't going to have good range, either.
If you're using this as a travel router or something like that, sure, it can be fine. But if it is for a more permanent installation (like at home), the Pi is better purposed purely as a router, and then you can add some purpose-built APs which will have much better performance (even an old 802.11n AP will outperform the Pi's built-in wifi).
Exactly, trying to use it as a travel router. Not going to use it often, and my Pi 4 will be sitting idle whenever I travel anyway, so thought it can be utilized just for this purpose without having to spend any extra cash.
2.4 just to see if I can get a little better range vs 5 GHz.
Actually read it before posting my question, but I thought they were referring to the "Allow legacy 802.11b rates" option. But no, there is actually a selection to change the wireless mode from "AC" to "N" or "Legacy". "N" doesn't work at all. "Legacy" does.
The signal quality is actually not that bad. I believe it is an adequate travel router to let a few laptops / mobile phones connect to the internet. Tried 2 simultaneous connection with no problem. Range is also ok.