Google/SEO specialist for indexation issues (Dutch website)

We're a Dutch company with a website. A little while ago Google and Bing indexed the largest part of our URLs but now suddenly it's been slowly deindexing without giving a reason. We're looking for an expert to help us figure out what's going on and solve the issues. We're talking about over 350,000 pages that offer localized tutoring services to students and high school pupils or offer summaries and practice assignments to students. Google/Bing is not telling us why it's not indexing our pages, so this task exists out of: Step 1: figuring out what's wrong Step 2: educating us on what's wrong Step 3: device a plan to fix what's wrong Step 4: autonomously fix the problem You'll be working with our main developer who's supervising and assisting with technical issues. We're looking for an expert who has: - knowledge of on-page SEO both content-wise and IT technical - experience with and knowledge of Google indexation and Bing indexation - the ability to work with WordPress and PHP Please note that we're in the Dutch time zone. We need a substantial overlap of working/daylight hours in order for this to work.