Customers can provide their own CSR on an OV Code Signing cert order via this link

Retail: https://secure.sectigo.com/products/!PlaceOrder?product=8
Partner: https://secure.trust-provider.com/products/!PlaceOrder?product=8&reseller=y&ap=partners-AP-code

note: replace partners-AP-code at the end of the link, with the Reseller AP code within their SASP account.

Technically, when the customer uses a specific link below to enroll for a code Signing certificate.
https://secure.trust-provider.com/products/!PlaceOrder?product=8&reseller=y&ap=Pending
Customer will not be able to use the collection email to acquire the certificate.

In-order for a customer to acquire public certificates, they can login to their account via
https://secure.trust-provider.com/products/frontpage?area=ssl
They will then be able to click the "Code Signing Certificates" and "Download as zip" button to acquire the public certs.

QUESTION: How did our client get access to the end user portal for exporting the certificate since this would have been placed through TheSSLStore’s account?

ANSWER: password reset can be completed for the customer on their account, since you create a separate account for the customer when enrolling for the cert.
The admin email address on the account is associated with the customer. Password reset can be done via this link https://secure.trust-provider.com/management/passwordResetRequest.html

QUESTION: If we’re able to grant access to the end user portal for our customers to download the certificate, how do we do it?

ANSWER: See above using the password reset method for the customer to gain access to their account.

QUESTION: Are we able to download the certificate from our current Reseller Portal as advised by support? How?

ANSWER: You won’t be able to login to the reseller portal to acquire the certificates.
However, the alternative for the partner to acquired and collect the Code Signing Certificates by using CollectCCC API
https://secure.trust-provider.com/api/pdf/reseller/customclientcertificates/CollectCCC%20v1.11.pdf

Please use the 'CollectCCC' endpoint as like here
https://secure.trust-provider.com/products/download/CollectCCC

There is an additional undocumented parameter and value
set 'product=8' to the call.
Use the code-signing order number.

You'll be able to download the signed certificate from there.