Log in to IONOS – you need three files:
- private key (
.key
) - certificate (
.cer
) - intermediate certificate (
_INTERMEDIATE.cer
)
If you don’t have the private key, Revoke Old Certificate, then Reissue Certificate – on Firefox this is a bit weird but it’ll download the key file but until you open it won’t complete the reissue.
In Plesk go to Websites & Domains > SSL/TLS Certificates > Advanced Settings > Add SSL/TLS Certificate.
Enter a certificate name:

Scroll to “Upload the certificate files”:

Browse each file (it doesn’t matter that they’re cer
rather than crt
), use _INTERMEDIATE.cer
for the CA certificate, then click Upload Certificate.