Only one connection, everything else fails

[LambdaEnt]

I am successfully connecting from one Mac desktop, using the native built-in client, to an off-site Windows 10 machine running SoftEther Server.

But nothing else can connect: No other Mac (built-in or SoftEther client), or iPhone.

Windows 10 native VPN connection, and Windows SoftEther client, both give the same error:
"Connection to the server failed. Check network connection and make sure that address and port number of destination server are correct"

User names, passwords, and pre-shared key are entered correctly. UDP 500 and 4500 are port-fowarded at the router to the server.

[drkrool]

Is it on the same network? If by any chance you are trying on another network as some cellular service, some cell provider block udp 500 and 4500

You can test if the port is reacheable by using telnet on the vpn's ip and port (in this example 192.168.3.1 on port 5555)

if opened :
telnet 192.168.3.1 5555
Trying 192.168.3.1...
Connected to 192.168.3.1.
Escape character is '^]'.

If closed :
telnet 192.168.3.1 5556
Trying 192.168.3.1...

[LambdaEnt]

No, it’s not on cellular. My iPhone, however, is on the same network (WiFi) and it won’t connect.

This has been attempted from several devices, on different internet providers.