I inserted a wireless N dual band M.2 NGFF combo card (TPE-M2NCRD) from ThinkPenguin into a Librem 5 to see what happens. Some websites indicate that the Atheros AR9462 chip could interface via PCIe and USB, so I had some hope that the card could be capable of USB and work in Librem 5 despite the stock Librem 5 Wi-Fi cards interface being SDIO.
However, it didn’t work out of the box and didn’t show up in lsusb output on Librem 5. Maybe USB was for Bluetooth in this chip. I didn’t test TPE-M2NCRD in the second slot in place of the cellular modem because the contacts couldn’t fit. The overall result is the same as with my previous attempt with another card: TPE-M2NCRD2. Both work fine under PureOS on a laptop but not on Librem 5.
FWIW, the default udev and usbguard rules aggressively turn the third USB port off as nothing’s expected to be there. You may have to tweak them if you want to use it. You’ll also have to keep in mind that the anti-reset quirk for modem PM won’t be compatible with wakeup-capable USB device present in the Wi-Fi M.2 slot.
Other than that, you’ll of course need a relevant driver to be present in the kernel config.
Since you mentioned that the card can use both PCIe and USB, you’ll also need to check its datasheet to determine how to select appropriate interface.
Generally speaking, using USB in that slot should be possible if you know what you’re doing, but it’s not a supported configuration and there are several limitations you’ll have to keep in mind. Even if you’ll get it working, you’re likely going to suffer from power management issues. It’s probably not worth doing at all unless you’re prepared for a really deep dive into device enablement.
If someone at Purism approached me and asked “can we get this USB M.2 card working for Wi-Fi on the Librem 5”, I’d strongly advice against doing that.
The issue with that one is that power saving depends on an external oscillator which the Librem5 does not have. Finding a wifi card with an integrated oscillator would be better
edit: sorry I just realized your issue was about using the USB interface instead of SDIO, and not about finding a wifi card
