Please fill out the fields below so we can help you better. Note: you must provide your domain name to get help. Domain names for issued certificates are all made public in Certificate Transparency logs (e.g. crt.sh | example.com), so withholding your domain name here does not increase secrecy, but only makes it harder for us to provide help.
There is nothing wrong with Let's Encrypt. You got a cert yesterday (see crt.sh here)
But, your server is not using the LE cert. It is using a self-signed Asustor cert. You can see that with a site like this SSL Checker. You need to find out why your device is not configured properly.
You might try searching this forum for 5401. Several people have reported this recently although I did not see any clear explanation of a fix. You might also try a forum for your Asus device.