Certificate chain is incomplete

[davidmcevoy@outlook.com]

Not sure what format Sectigo delivers but you you can check using information on the internet.
Use a texteditor to examen the certificate file. Does it start with -----BEGIN CERTIFICATE----- Mine did. 

I had to rename my certificate to cert.pem and combined the root and intermediate certificates into the file root.pem. Just with a texteditor in the format I described in my previous post.
The root and intermediate certificates are sometimes packed into a zipfile called root.bundle or alike.

SSLbudy is a tool that I didnt use in a long time.

Oh that is interesting, might give it a try.  Currently SSLBuddy creates two files, .pem and .key, do I just need to edit the .pem file and key the .key file as it stands?