How to connect to a VPN server through NAT from a smartphone with Windows Phone 8.1?

Question

VPN

I'm trying to set up a VPN connection on a smartphone with WP 8.1
VPN server - RRAS on Windows Server 2008 R2, L2TP with a key, without a certificate
UDP 500, 4500, 1701 forwarded on the router

• Clients with Windows 7/8 connect both from inside the network and through NAT
  
• iPad connects both from inside the network and through NAT
  
• WP successfully connects from within the network but fails to connect through NAT - error 809.
  

The NAT problem is described in this article: https://support.microsoft.com/en-us/kb/926179 , but it is only for older operating systems
. To check, I tried to set up a connection with Windows Server 2008 (not R2) - the connection was established after adding the parameter AssumeUDPEncapsulationContextOnSendRule and reload; before editing the registry - error 809
Tell me what needs to be done so that Windows Phone 8.1 clients can connect through NAT, what am I missing?