The error states just that the site doesnt fully support IPv6, What does your Browser state when trying to visit the site, cause for me it works fine on my KPN connection.

Also you Could try a Pingplot to the website or possible a Tracert.

just open CMD in windows and run "tracert enlinado.pl" command

But problem is that for me it was also working fine for half hour or something like that. That’s why I’m sure it is something related to my KPN router. I’m at work now and everything is working perfect. On mobile data also. Below you can see tracert but it is from my work connection so I’ll repeat it when I’ll be at home and I,ll share it to you. Thanks for response.

Ok here is from my experia at home:

Hello @Hadex with my KPN Box 12 modem I can reach the mentioned websites www.enlido.pl and www.kursy.enlynado.pl with no problems and I use IPv4 and IPv6.

CC: @LaurensZalm 

Ok here is from my experia at home:

 

 

Well thats rather interesting considering mine looks exactly the same just diffrentiating the first 2 IP's all the other responses are exactly the same and for me it works perfectly, if your router would indeed not pass it through the routing would be completely suspended after the "router”aka the first IP. 

So this means it does pass through the router in working order ​, could you show a Printscreen of what happens if you try to open the webpage ? ​​​​​​

Ok thank you all, issue is solved… my activity was not properly recognize and their firewall block my IP address at all. After contact with admin enlinado everything is working well again.

Ah they blocked your IP i see… 

Well you got it working so that was the goal