I followed a post here (ER605: How to setup a Client to LAN IPSEC VPN to connect to via Windows 10 for example?) and this TP-Link FAQ article (How to establish an L2TP Server by Omada Gateway in Standalone mode), but my Windows 10 PC shows an error message, as shown below:

The network connection between your computer and the VPN server could not be established because the remote server is not responding. This could be because one of the network devices (e.g, firewalls, NAT, routers, etc) between your computer and the remote server is not configured to allow VPN connections. Please contact your Administrator or your service provider to determine which device may be causing the problem.

However, I can see on the ER605's syslog screen these messages:

1 2024-12-14 00:16:02 IPsec NOTICE WAN: Phase 2 of IKE negotiation succeeded. (Peers=10.1.10.2<->55.66.77.88)
2 2024-12-14 00:15:52 IPsec NOTICE WAN: Phase 2 of IKE negotiation succeeded. (Peers=10.1.10.2<->55.66.77.88)
3 2024-12-14 00:15:44 IPsec NOTICE WAN: Phase 2 of IKE negotiation succeeded. (Peers=10.1.10.2<->55.66.77.88)
4 2024-12-14 00:15:40 IPsec NOTICE WAN: Phase 2 of IKE negotiation succeeded. (Peers=10.1.10.2<->55.66.77.88)
5 2024-12-14 00:15:38 IPsec NOTICE WAN: Phase 2 of IKE negotiation succeeded. (Peers=10.1.10.2<->55.66.77.88)
6 2024-12-14 00:15:37 IPsec NOTICE WAN: Phase 2 of IKE negotiation succeeded. (Peers=10.1.10.2<->55.66.77.88)
7 2024-12-14 00:15:37 IPsec NOTICE WAN: Phase 1 of IKE negotiation succeeded. (Peers=10.1.10.2<->55.66.77.88)
8 2024-12-14 00:15:37 IPsec NOTICE WAN: IKE negotiation began in responder mode. (Mode=Main Mode, Peers=10.1.10.2<->55.66.77.88)

(Where 55.66.77.88 is my home router's IP address from the ISP.)

Some more details: The ER605 is connected to a WAN router from the ISP (Comcast). So its WAN IP (10.1.20.2) is not a public IP address. However, I did set up port forwarding of these 3 UDP ports to the device:1701, 500, and 4500.

Am I missing something? Thank you in advance for any/all help.