Hi Gertjan, thanks for your reply,
Here are 2 sites that are unreacheable:
https://www.theandroidsoul.com/how-to-backup-apps-and-data-without-root-using-helium-android-app/
https://techcrunch.com/2016/04/19/the-first-comprehensive-study-on-women-in-venture-capital/
https://www.emacswiki.org/emacs/FoldingMode
At first I thought it had to do with https, but that's not the case, other https sites load fine.
MTU is default: BLANK
I'll try to look into DNS, as I have thought about that being the culprit at the root of the problem, I think that's also what lead me to adding Google's DNS servers.