Access issue for the records once there is a change in object permission

Dear Team,

We are facing an issue with the access rights for the records in the contact and account. By default, in the object permission for the contact and account, all the access are provided to the system admin and all employees and later had added the portal user in the operational permission and along with few organization roles and had provided the access as shown in the attached screenshot [Object_Permission.png] for both contact and account 

 As the portal user was not able to view the record even though the access was provided, we did disable the use operation permission and updated the record permission by doing that all the access for the record was removed. 

Later, again we enabled the use operation permission and updated the record permission, but the access wasn’t provided to the records which was removed from the previous step [Record.png]

There are more than 1 Lakh record in both the section, how can we provide back the access to all the records from the object permission?