Linux - Linux sync fails, Linux - Android works

Description of the issue:
I have a Linux Fedora 33 desktop, a Xubuntu 21.04 laptop and an Android phone all running Brave 1.27.74. The laptop and phone are on a sync chain which I created on the desktop but the desktop never joined the chain. Note that the desktop has a very aggressive firewall which might be blocking sync.

Actual Result (gifs and screenshots are welcome!):

On the laptop and phone chrome://sync-internals show “Transport State Active” and the expected log of update requests. On the desktop, it shows “Transport State Disabled” and “Disable Reasons Not signed in” (which I think is now obsolete), and clicking Request Start has no effect.

Expected result:

Expected the desktop to join the sync chain, but no joy.

Reproduces how often:

Every time. I tried creating the sync chain on the desktop then having the laptop join several times but got the same result every time.

Additional Information:

It’s very possible that the firewall on the desktop is blocking something important. I looked everywhere I could think of for details of the sync protocol and port but nothing turned up. If the protocol uses TCP:443 like a secure web request, then the firewall is not the problem and I need to look elsewhere. If the protocol uses something else then I need to poke a hole in the firewall.

On the desktop, when I go to Sync>Settings>Manage your synced devices I don’t get the prompt asking whether I already have a sync code, presumably because that’s where I generated the sync code that the chain of my laptop and phone is now using, so I don’t have an opportunity to join that chain.

The laptop and phone both show both devices on the sync chain.