Problem accessing (certain) web sites

I recently decided to switch my ASUS RT-AC66U router from Asuswrt-Merlin because: 1) it is no longer supported / receiving updates, and 2) I started not being able to access certain web sites.

Ikea, for example, gives me the following error message:

Access Denied

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You don't have permission to access "http://www.ikea.com/" on this server.
Reference #18.292c1ab8.1574976708.377acce4

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While other web sites give me a 404 error.

I installed...

Problem accessing (certain) web sites