Based on the recommendations of the good folks behind Home Assistant, I did get myself a Zigbee 3.0 USB dongle.

This one:

• ITead SONOFF Zigbee 3.0 USB Dongle Plus Model “ZBDongle-E” (EFR32MG21 variant)

All looked good, until I got the request to enter the Zigbee sierial port device ...

So, your QNAP runs a Linux variation and how am I supposed to detect this "com port" device?

Not knowing where to look, I first tried using "lsusb", which didn't help much besides seeing that the USB device is connected.

Before connection the USB dongle (removed some irrelevant lines):

[~] # lsusb
...
Bus 005 Device 001: ID 1d6b:0003 Linux Foundation 3.0 root hub
Bus 003 Device 011: ID 1058:2667 Western Digital Technologies, Inc. 
Bus 002 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 001 Device 003: ID 1005:b155 Apacer Technology, Inc. 
Bus 001 Device 002: ID 0438:7900 Advanced Micro Devices, Inc. 
...

After plugging the dongle in, it showed this:

[~] # lsusb
...
Bus 005 Device 001: ID 1d6b:0003 Linux Foundation 3.0 root hub
Bus 003 Device 011: ID 1058:2667 Western Digital Technologies, Inc. 
Bus 002 Device 004: ID 1a86:55d4 QinHeng Electronics 
Bus 002 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 001 Device 003: ID 1005:b155 Apacer Technology, Inc. 
Bus 001 Device 002: ID 0438:7900 Advanced Micro Devices, Inc. 
...

The "QinHeng Electronics" device (Bus 002, Device 4) is the dongle - so it did get connected. No clue to the device name though.

From reading up on the topic, a device could show as /dev/ttyUSBx and /dev/ttyACMx. (x= number, starting with "0" - difference explained in this article if you're interested).

So here we did the "before-and-after" trick again, just by listing the devices that start with "/dev/tty", and comparing that with the lits of these devices after we connect the dongle.

With the dongle NOT plugged in, pulling a list of all tty devices:

[~] # ls /dev/tty* > /tmp/ttydevs_before.txt

To make sure I know how many there are, I did a count as well:

[~] # ls /dev/tty* | wc -l
69

Next I plugged in the dongle - give it a few seconds, maybe try with "lsusb" to see if the device appears.
Or try it with count "/dev/tty" devices (69 became 70! Woohoo!):

[~] # ls /dev/tty* | wc -l
70

Once the device appears, pull another list of "/dev/tty" devices asn:

[~] # ls /dev/tty* > /tmp/ttydevs_after.txt

No compare the lists:

[~] # diff /tmp/ttydevs_before.txt /tmp/ttydevs2_after.txt   
 --- ttydevs_before.txt
 +++ ttydevs_after.txt
@@ -63,6 +63,7 @@
 /dev/tty7
 /dev/tty8
 /dev/tty9
+/dev/ttyACM0
 /dev/ttyS0
 /dev/ttyS1
 /dev/ttyS2
[~] # 

Here we compare ttydevs_before.txt and ttydevs_after.txt and we see that in ttydevs_after.txt a line is added (in comparison to ttydevs_before.txt):

So the device we were looking for is ...

/dev/ttyACM0