Time Machine is not (yet?) supported by Syncthing because it does not use UPnP. Thought I would document how I set it up on our home network. Much help from How to Configure Port Forwarding on a Time Capsule
Give one machine a fixed internal IP
I set my primary machine receive the same internal IP address. On the Time Capsule, assign a DHCP ID to an IP. In my case revelation will always be 10.0.0.36.
Click the + button to add a DHCP Reservation:
Make sure your Mac is configured to ask for the correct Client ID, revelation in my case. Look under the Advanced button on the Network preferences.
Forward an external port to an internal machine
Back to Time Capsule. Now add a Port Settings entry mapping the Syncthing port (22000 by default) to an internal IP:Port combination. Click the + under Port Settings.
In my case, I want TCP and UDP from the Internet coming to port 22000 to be routed to 10.0.0.36:22000. Note: 10.0.0.36 is the IP assigned in the previous step.
Now Syncthings running on other machines will connect to the Time Capsule’s Internet-facing IP address (assigned from an ISP) to port 22000 and the Time Capsule will forward to an internal IP on port 22000.