Devices in network not using Pi-hole

[deHakkelaar]

@lonkelo

   dns-server: 192.168.2.3
   dns-server: 192.168.2.254

Your router with IP 192.168.2.254 is advertising its own IP for DNS to the clients alongside the Pi-hole IP 192.168.2.3.
This means that clients can bypass Pi-hole:

Why should Pi-hole be my only DNS server?

Reduced Ad Blocking Capability The main reason you should Pi-hole as your only DNS server is that you will see increased performance in the blocking of ads. If you have two DNS servers (Pi-hole and something else), your network clients may not always query Pi-hole for name resolution. If a query happens to be answered from a non-Pi-hole DNS server, your block lists will not apply (since that DNS server doesn't know about them). All Queries Need To Go Through Pi-hole First Since other DNS se…

If you cant get the routers DHCP service to only advertise the single Pi-hole IP for DNS, you could opt to let Pi-hole take over DHCP services for your LAN:

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. …

EDIT:

@lonkelo

but not adds on Youtube

Oh Youtube ads cant be blocked as the ads are coming from the same domains as where the content is coming from.
A browser ad blocking plugin can fill that gap or get a Youtube premium subscription.