If you find that on your ADSL/VDSL connection, you are unable to access some particular website, you can refer this document.
Some specific site/s is inaccessible from a PC behind the DrayTek router. You are able to resolve domain names with IP addresses and you are not getting any error message in web syslog.
Also you have not configured any web content filter or firewall policy to block those sites.
This issue sometimes can be caused by large MTU size.
You can go to WAN>> Internet Access
Reduce MTU size to lower than default value and check
Click Ok to save the settings, reboot the router and check the website now.