Fluctuations in SERP Rankings! But why?

Last month, I started a campaign on a blog post targeting a medium keyword difficulty. After two weeks, it began to rank but was positioned at 19 on the second page. After four weeks, it moved to the first page, reaching the 6th position. However, it has now slipped back down to the second page. In addition, I have been building links, focusing on the main anchor text and acquiring high Domain Rating (DR) links, but I'm still in the process of doing this. Throughout the campaign, I chose the main keyword as the anchor text for these links. Now I would like to ask if there are any issues with my approach and what I can do in the future to improve my rankings.