Router refusing connection to one website / domain

So yesterday I tried to connect to my ftp host via FileZilla and Adobe Muse a few times, after that did not work I think the router somehow blocked any connections to that one website.

If I connect to the website via 4G it works fine, both on iPhone and macbook. via wifi its not working on either.

I tried connecting to the internet via ethernet (not via the Airport) and that worked fine. So I'm sure it has something to do with the Airport Extreme.

What I've done to try fixing it:

Restarted router,

Factory restored router

Changing settings (DNS, NAT...)

Have you experienced this, and have you solved it?