What tools can I use to check to see why my domain can not be validated. datalabmi.com
I have check the Certificate Transparency logs (e.g. crt.sh | datalabmi.com)
My domain is:datalabmi.com
I'm using Synology DSM 7.0.
A browser should give an explanation. But, I often use a test site like this SSL Checker. Or, try the Server test at SSLLabs.com
Using those, I see the problem is your server is using a self-signed certificate instead of the one you created with Let's Encrypt. You should refer to your Synology docs on how to configure that.