In such a large community which is Live for so many years, there is no one using Sectigo code signing successfully ?
If there is one - Kindly tell me - what is the timestamp server you are using in Install shield ?
First you should change the timestamp server details in settings.xml file available in installshield system folder
Then you can import your certificate to trusted root location, once you imported,
https://community.flexera.com/t5/InstallShield-Knowledge-Base/How-to-add-certificates-to-the-Trusted-Root-Certification/ta-p/4118
Try to select the certificate from certificate store, and select the options (refer attached screenshot for importing from store)
Also refer similar community post:
https://community.flexera.com/t5/InstallShield-Forum/Invalid-Timestamp-http-address/m-p/179892#M69577
