Thanks @Andrey Yasko for the reply, I transferred all the content to UNA and also added a lot of new content made by members and me, I redirected the most popular backlinks to a new URL, made a new sitemap in the paid sofware, it just doesn't help! Still 20K pages not indexed, and only 5K are indexed. I regularly check the search console and fix it. I've been fighting this for eight months and I'm slowly losing my will.