I start jack using qjackctl or cadence,
from within either qjjackctl or cadence, I select an input device from the jack settings / configuration section,
(do you see your audio interface and can select it as input device)?
if so, then from the same location (jack configuration block) select how many channels you want to use,
if your device has two, select two for in,
for channels out, I leave default so I can play the recorded sound back through my laptop for testing,
if I wanted to hear the playback through monitors connected to my external audio device I would select the device as output
and again select the correct number of channels to use.
Then I start Ardour, which gives me the choice of JACK AlSA and I think pulseaudio, I Select JACK,
Is this how you are approaching setting up Ardour.
Once Ardour is running, then you need to add a track, mono for a mic,
then go to mixer on the far top right, open mixer, select input for the track you created,
and select one of the channels from you audio device, if all went ok, there will be listings there such
as in 1, in 2, in 3 etc, so select the one your mic or instrument is connected to,
arm your track, arm record in the top bar, if the levels from you audio interface are set high enough
you will see the track meter moving because of local noises, speak or play your instrument and the meter will
of course rise accordingly, now hit play, and recording will start.
I can either start my laptop with the usb interface connected, or connect the usb interface after I start the computer,
either way, I have the interface connected to my laptop before I start jack, or Ardour.
In some cases I have heard of people need to reboot with the interface connected, I never have to.
Hope this helps