I don't see any related errors in the system logs. I'm not sure if this message is regarding the let's encrypt certificate, but in the logs I do see a message saying, "httpd: Succeed to init SSL Certificate...80".
Hello,
I set up a DDNS service through the Asus server and chose to use a let's encrypt certificate. The router tells me that the certificate is active, but when I connect to the DDNS server, I get messages informing me that the connection is not secure. My router is RT-AC3100.