Connection issues are usually caused by Firewalls or VPNs, or APs running in Isolation Mode. You may see an error like:
POSIXErrorCode(rawValue: 60): Operation timed out
Firewalls
You must allow Strongbox to accept incoming connections if it is acting as a Source device. Often your firewall can come automatically enabled and blocking Strongbox by default. You will need to allow Strongbox access through the Firewall to use Wi-Fi Sync.
NB: You will need to restart after updating the firewall.
VPNs
If you are running a VPN this can interfere, but there is usually a setting to allow local network traffic while preserving your privacy.
AP in Isolation Mode (Hotel/Cafe/Public Wi-Fi or Eduroam)
If you are on a network where the administrators have decided to run in Isolation Mode this will likely cause issues. Isolation Mode basically means that your device cannot "see" or communicate with other devices on the same network. This can be the case on large corporate networks. It can also be the case in hotels, cafe's, restaurants and other public networks. We have also come across an instance of this on the Eduroam academic network, though it's not clear if that is system wide or just within a particular institution. Feedback welcome.