A question.
Is it possible to syncing 2 computers who are in 2 different local-networks?
In the same local-network it works and they can see each other, but when I’m moving 1 to a different local-network, then the connection is lost.
Can I make it working anyway?
Depends on which discovery options you have enabled and if the individual local networks are reachable from one another and/or if one of the relay-servers is reachable.
If the networks are reachable directly, you can configure fixed IP addresses or DNS names.
Or you can have Global Discovery enabled and then it should with UPnP and fallback to relays start to sync over the Internet. Depending on your firewall settings of course.
If you leave it for a few minutes, it should start working automatically (so long as you’re using version 0.12), provided that Syncthing has access to the wider internet. Make sure that at least one of the nodes displays “Relays 1/1”.
But you will be taking a performance hit for not connecting directly, so I suggest you somehow allow accessing one lan from the other in order to establish a direct connection.