I might be wrong but Startpage should not be blocked any more than Google, DuckDuckGo, Bing, Ecosia, Yahoo, etc. are.
It's blocked because they don't support safe search as URL.
You can write them a message fixing it.
Until, just disable the NextDNS feature.
DynamicNotSlow Ah. That makes sense then. I added them to my allow list to get past the issue.