Can't see 5.5 GHz WiFi on raspberry pi 4b

[aweb]

I have a raspberry pi 4b. I have uncommented the line `WIRELESS_REGDOM="US"` in `/etc/conf.d/wireless-regdom`. Unfortunately, i am completely unable to connect to an AC-only wireless network at 5.5 GHz (channel 100). The same `wpa_supplicant.conf` file works fine on an x86 laptop. However, on the raspberry pi 4b I don't even see my SSID in the output of `iwlist scan`.

I live in a pretty crowded area with a lot of APs. However, when I run `iwlist scan` on the pi 4b, I only see AC channels 40, 48, 153, and 157. By contrast, the laptop can also sees SSIDs at channels 100 and 116 (including mine at 100).

One possible clue may be that `iw reg get` looks different on the two machines. On my x86 laptop, I get two sections, global and phy#0:

```
global
country US: DFS-FCC
...
phy#0 (self-managed)
country US: DFS-UNSET
```

By contrast, the raspberry pi has only the global section.

Can anyone tell me what I might be doing wrong? Thanks.