Successfully enough that I’ve never needed to fiddle with it 
I wasn’t sure what to download from your link, so first I tried downloading the SectigoPublicServerAuthenticationRootR46_USERTrust.crt linked here:
I simply double-clicked to open it in Keychain.app. That was all it took to install. Now Safari trusts the certificate, the support assistant has its content back – and DT’s markdown clipper works again.