If you have been using Firewalla VPN Server for a while, and suddenly the VPN client cannot connect to the Server, there might be 2 problems causing this issue.
Problem 1: UPnP port forwarding was wiped by the router.
Solution: Turn off the VPN and then turn it on after 5 seconds. This will refresh the UPnP port. If this doesn't work, please reboot Firewalla. If you know how to configure the router, the best way is to insert a static port forward in your router
Problem 2: VPN profile is expired.
There is a bug that VPN profile may incorrectly expire after 30 days, this bug only occurs when creating a VPN profile for the first time.
Solution: Reset VPN profile
Step 1: Open the box main page in Firewalla App, tap on VPN Server button -> Setup.
Step 2: Tap on the blue button "Tap here to reset the profile and password" -> Confirm.
Step 3: Download the VPN Profile, import it to your VPN Client, input the new password when asked.