Can't access /admin (another webpage running there already)

[deHakkelaar]

You can try see how the blockpage looks like by browsing to below link:

http://doubleclick.com:8899

@Paul-v

My ISP’s supplied router for some reason doesn’t want me to set a different DNS

Can replace routers DHCP with Pi-hole's one:

How do I use Pi-hole's built in DHCP server (and why would I want to)?

Using Your Existing Router For Network-wide Ad Blocking You might not need to use Pi-hole's DHCP server: In many home environments, your router also functions as your DHCP server. In this case, you can often set Pi-hole to be the DNS server for your network clients in the router's DHCP (or LAN) settings page, which allows all of your network clients to block ads simply by connecting to the network. Setting it up this way is also what makes Pi-hole very powerful for network-wide ad blocking. …