How to use Agile on TaaS environment

How difficult it is to implement agile methodology when the customers are not directly involved.
ITeste is a company specialized in Software Testing and has started its activity with the purpose of providing exclusively testing service to software development companies. Over time, we realize a need to shift our focus to serve software buyers companies. In this change we realized that we had to change the way we work, because the objectives of our current clients were different. The software came to be homologated by the customer with many basic defects. As the goal of providing a better service we developed a TaaS methodology to better serve our end customer. At this moment our biggest challenge was how to work agile to anticipate the encounter of the defects being that often we did not have access to the company developer and sometimes even the user. We only received the software product ready for homologation. To solve this problem, we internally developed a methodology using agile practices to jointly with our client to seek a successful result at the time of homologation.