As another data point, my iPhone (iOS 17.5.1) and my iPad (iPadOS 17.5.1) are both able to establish outbound L2TP VPN connections even when the Deco's L2TP VPN server is enabled. So the issue may be specific to the native L2TP VPN functionality built into Windows 11. I don't have other VPN client software to test right now.

  @David-TP 

Thanks for the reply. The only Windows devices I have available are two Windows 11 systems (no earlier Windows versions unless I use VMs), and I can reproduce this issue on both of those devices.

The remote L2TP VPN server is a Ubiquiti UniFi Dream Machine Pro, running UniFi Network 8.3.32, which is the latest available as of this writing. There isn't anything complicated about the L2TP VPN server setup there. All I had to do was enable VPN Server, select L2TP, provide a pre-shared key, and create username/password credentials. It's very similar to the VPN server setup experience on Deco, actually. The UniFi Network interface does have some other options available under the Advanced section, but I have that set to Auto, so all of those changes are grayed out.

That UniFi VPN server configuration allows me to connect from Windows and iOS/iPadOS clients, except in the scenario I have described in my earlier posts.

Should I still send this to the email address you provided?

Quick update. TP-Link Support contacted me to perform some remote control troubleshooting, complete with Wireshark packet captures, SSH sessions to the main router and satellite unit, etc. So far we've had two such sessions, each lasting more than 2 hours -- but they were productive!

The first session confirmed that this was indeed a bug, and the tech found a temporary configuration workaround, which appeared to be disabling hardware acceleration/offload. The second session included TP-Link's R&D engineers, who did further digging and tweaking to isolate the underlying cause. At the end of the session, they said they now understood what was going on, and expected to have a patched firmware for me within a few days. If that worked out, they would plan to incorporate that fix into a future production firmware release.

Have to say, as my first experience with TP-Link's products and their support, I am extremely impressed. I spent 15 years in various Windows IT roles, so this isn't my area of expertise, but I know knowledgeable techs and competent troubleshooting, and dedication to the task when I see it. And I definitely saw it with TP-Link.

Special thanks to Parker at TP-Link Support! :)

The test firmware that I received solves this issue.