Hi, are you connecting with the exact same (external) URL as the certificate is valid for?
eg , myname.synology.me
if you are on an internal network, and connect on the local range eg 192.168.x.x you will get a certificate error, as the lets encrypt is only valid for the external domain name.
webdav uses a custom port on most synology devices, i think 5006 for SSL, so be sure you are specifiying the port when connecting. https://myname.synology.me:5006
Im going from memory here as i dont have webdav installed on a Syno right now, but hopefully this helps.
I use also ssl certificates ( QNAP NAS ) and when I use it internally is have added hostname.domainname in hostfile to point to internal IP addres of NAS.
Now you can use hostname.domainname also internally incl. encrypted connection
When using outsite the door, I flip my hostfile without this entry.
Now you work with hostname.domainname externally.