Server behind NAT, Android problems

[Starlight5]

I have a RPi2 running SoftEther VPN. It is behind NAT, ISP not being cooperative to open anything. I have the following situation:

VPN works when connecting from Internet via Windows Client, using SoftEther DDNS name;
VPN works when connecting from home network via built-in Android VPN, using server local IP address;
VPN does NOT work when connecting from Internet via built-in Android VPN, using SoftEther DDNS name.

Logs register nothing when Android/iOS device is trying to connect from Internet, it seems they don't reach the host at all.

My configuration is:
TCP port 443 listening in server, and forwarded to server on my router;
UDP ports 500 and 4500 are forwarded to server on my router;
IPSec/L2TP - all enabled;
VPN over ICMP/DNS - disabled.

Internet connection used for tests:
3G mobile (using my Android phone to either connect itself, or share connection with Windows laptop, and by iOS phone to connect itself).

Please help me. =\

[WarptecAU]

If it works internally, it means it is a port blocking / forwarding issue.

Try forward Port 1701 UDP

[thisjun]

I think ISP on server side or client side blocks L2TP/IPSec.