first thing I can think of is that you've screwed up the URL structure. This is one of the easiest things to fuck up when you move / migrate / recreate a website, pages that used to rank well get their URL changed (even if by just one character, it doesn't matter) and google - rightfully so - considers it a different URL so it doesn't rank it anymore since the original URL has probably also had authority and trust assigned to it (no idea what trust means in this context so please don't ask me because I don't know, all I know is that google does assign trust - whatever this means - to URLs, I've heard it a few years ago from more than 1 person)
Something else I can think of that could be at fault - especially if the URLs have changed - is that they've lost some / all of the good backlinks that were pointing to them.
So, go through each important page (that you remember used to rank well) and check whether all (internal and external) links that were / are pointing to those pages have been lost / nofollow'd / removed / broken / whatever, and fix them (if possible). You can use a site auditing tool like Screaming Frog to check for broken (internal) links, for external links I guess you can use GSC or tools like Ahrefs / Scrapebox to find all of the links and check their liveliness / indexation status / anything else that matters...
from what I know the hosting can impact the rankings too, so yeah... it might be the hosting too but not in the sense that it's bad hosting but - again - in the sense that it's new hosting so you might have screwed something up during the migration
Unfortunately, I'm not tech-savvy enough to give detailed instructions on what to do, but there have been lots of cases similar to yours (this year alone I think I've read about 5 or 6 similar cases) on the forum, so take some time to look them up, maybe you find tips you can use to fix it...
