Fwd: Monitor mode (w. TP-Link WN722N & Fedora)

[Fulko Hew]

F.Y.I. I also sent this to the Wireshark user's list to see if someone
there can help me.

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I'm trying to sniff my Wifi for a project, so I purchased a TP-Link WN722N
USB adapter
and followed the various instructions I've found on the Web.
(I don't know if it's a Wireshark issue, a driver issue, or a Fedora issue.)

ip link set wlan0 down
iw wlan0 set monitor none
ip link set wlan0 up

When I plugged the adapter in I saw a new device 'wlp0s20f0u3' in addition
to my internal adapter 'wlps20'.

Using the instructions, it failed:
# iw set wlp0s20f0u3 down
# iw wlp0s20f0u3 set monitor none
command failed: No such device (-19)

-->   I don't know why?

So now that I have 2 adapters, I enabled the TPLink as my network
connection (successfully) and tried to put my on-board adapter
into monitor mode, instead:

# iw wlp2s0 set monitor none
# iwconfig
wlp2s0    IEEE 802.11  Mode:Monitor  Tx-Power=22 dBm
          Retry short limit:7   RTS thr:off   Fragment thr:off
          Power Management:on
# iplink set wlp2s0 up
#

It looks good, so I start Wireshark (v 2.28) (Fedora 26),
and look at Wireshark's 'capture options' table, it shows the interface,
but under the heading 'Mon Mode', it shows disabled.
(I also have 'Use promiscuous mode on all interfaces' enabled.)

--> Why does it show disabled?
--> Is it really disabled ?
--> How do I know which frequencies (or bands) it might be listening on?

I started sniffing, but it doesn't capture anything.

... And after about 2 minutes, I get a pop-up error message from Wireshark
saying:
 "The network adapter on which the capture was being
  done is no longer running; the capture has stopped."

Where to start ?

Thanks
Fulko