Hi Rib,
when you have the certificate as a PEM- or CER- file available, you can open it with Windows.
In case of a PEM-file rename to CER, as otherwise Windows will not recognize it as a certificate.
In the last tab there is the ceritificate tree displayed.
Share us this certificate tree, it should contain at least 2 entries, server certificate and root CA.
Handshaking on secured connections should always start with the highest protocol available (TLSv1.2) and when this not accepted it will try the next lower version until it either hits an accepted version or no more versions available.
Ideally TLSv1.2 should be supported and accepted for most connections.
You can leave TLSv1.1 active as a backup protocol
When one of your partners only support TLSv1.1 they are hereby encouraged to check with their support to update the installation to support TLSv1.2.
Regards,
Holger