I am experiencing the exact same problem as https://community.tp-link.com/us/home/forum/topic/203410.

Since last 3 days, VPN stopped working for unknown reasons. I updated the firmware after it stopped working, restarted the routers across but nothing changed. The mac was connected on Wifi. I had not done any special setting in my initial setup other than defining the name of the network!

Lastly I switched security from WPA ->WPA/WPA2 and even before I saved the config then it suddenly started working. I am not sure if this was incidental or actually the fix and I dont know if it will stop working again.

So I request from the TP team - do you know the root cause of why VPN stops working randomly?