Affected ever since the last update. What should I do?

I have a PBN that consists of 100+ domains. Ever since the last update, it simply slide down and not on page 1. I am not penalised as some of the keywords went down to page 2 but not to page 1. I have tried to purchase my usual SEO service but its not working. Google have changed its algorithm. PBN doesn't seem to be working that much anymore. What are the new techniques that I need to incorporate for me to push up my usual keywords to page 1?

No specific advice to give you mate. Maybe you should clean up those links and start over. It's gonna take 6 to 18 months at least to recover IME.

A HQ strong PBN will always do the work,
You should aim for a better quality if you want to get decent long-term results,
The same goes for any type of links nowadays, including guest posts/niche edits, web 2.0s, crowd links, etc...