Recover Google rankings after overwriting one page with another and getting the duplicate content excluded

One of our pages was on page 1 of Google for a very important keyword. Some mistake from our developer team caused a disastrous issue in indexing that page. The parent page and the child page used the same content for a few days by mistake. Google then removed the child page from its index and kept the parent only, saying "Duplicate, submitted URL not selected as canonical".

I understand the issue. But I don’t know how to solve it! I have already brought back the original content on both pages and asked Google to reindex both pages through Google’s inspector tool in Search Console. But it doesn’t work! Google doesn’t index the child page anymore which is really crucial for us. Google has identified our child page as a duplicate of the parent page. I have re-written the whole content of the page and asked Google to re-index it again. No luck yet.

Do you have any solution for this issue?