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.