KDE Connect on Pixel phones, but no pairings?

[Jeffrey Walton]

Hi Everyone,

I'm having trouble configuring KDE-Connect on a couple of Android
phones and a desktop computer. KDE Connect is installed on all three
machines.

One phone is an older Pixel 4a, the second phone is a Pixel 7a. The
desktop is a modern Dell. All equipment is owned/operated by me. The
two phones are tethered to the desktop via USB. Here is some more
specific info:

* Pixel 4a
   - Android 13
   - No Superuser apps
   - USB-C tethered
   - Developer mode, USB Debugging
   - USB Pref, File Transfer/Android Auto
   - Wifi, 192.168.1.151/24

* Pixel 7a
   - Android 13
   - No Superuser apps
   - USB-C tethered
   - Developer mode, USB Debugging
   - USB Pref, File Transfer/Android Auto
   - Wifi, 192.168.1.76/24

* Desktop
   - KUbuntu 22.04.3 LTS
   - plasmashell 5.24.7
   - wired, 172.16.2.10/12

192.168.1.x/24 is hanging off my ISP's cheap router network segment.
My 172.16.x.x/12 network hangs off it, too. But the 172.16 network is
behind a pfSense firewall and everything is wired.

But there are no pairings, even on the phones in the same [wifi]
subnet. And clicking "+ Find devices..." does nothing on the desktop.

I've visited <https://userbase.kde.org/KDEConnect>, but the
troubleshooting steps have not been helpful. It looks like the Android
phones have not opened a firewall port:

# From the desktop machine to an Android phone
$ netcat -z -v 192.168.1.151 1714-1764
netcat: connect to 192.168.1.151 port 1714 (tcp) failed: Connection refused
netcat: connect to 192.168.1.151 port 1715 (tcp) failed: Connection refused
netcat: connect to 192.168.1.151 port 1716 (tcp) failed: Connection refused
...
netcat: connect to 192.168.1.151 port 1762 (tcp) failed: Connection refused
netcat: connect to 192.168.1.151 port 1763 (tcp) failed: Connection refused
netcat: connect to 192.168.1.151 port 1764 (tcp) failed: Connection refused

Clicking the hamburger (☰) and three dots (...) on the Android phones
does not offer an option to configure the device's firewall. But given
the phones are tethered to the desktop, it may be easier to just use
the tethered connection.

Does anyone have an idea how to get the devices paired to the desktop?

Thanks in advance,

Jeff