Certificate upload not recognized - host server

My host server does not recognize the certificate and the private key that I have created for my domain " linkshub.tech".
The problem started when setting up my WordPress dashboard; -setting - general, on WordPress directory link, HTTP I added s to HTTPS and saved changes, then my admin area couldn’t load back, and error 521 appeared, the web server is down, stating that linkshub.tech host error.
Contacted my host they couldn’t help, they wanted me to generate CSR and send it back to my Certificate issuer because my certificate was not recognized by their system, or it didn’t contain all required information, like CA’s Certificate Authority.

What CA issued your certificate?