Every year I try Syncthing with classroom computers in different School Districts and Syncthing gets blocked 90% of the time and Resilio AKA Bittorrent Sync gets through 95% of them. Now that Resilio wants $360 a year I really can’t justify the extra $320 a year for busting through firewalls.
Has Syncthing caught up on this feature in the past year. (I am not traveling to the various sites for the next few months so I can’t test it right now.
Relays help a lot with establishing connections in restricted environments. However, this feature was released quite a while ago, so maybe you’ve already tried that
There is experimental udp transport support available, which tries to punch through nats, but it’s not enabled by default.
You can try that by adding kcp://:22067 to listen addressed on both sides yet I am not sure if it will help as you haven’t quantified what the firewall is preventing exactly.