Ok. This makes sense a little bit.
Here's the logic on how you would have to proceed from here:
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Switch the class of IPs in the ISP router from 192.168 to 172.16 ( 172.16.0.0 to 172.31.255.255) under the Router's DHCP server settings.
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Install a low resource OS on your device that will host the Pi-hole installation (Maybe XUbuntu ?).
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Install Pi-hole on your (XUbuntu) device.
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Once installed, go to http://pi.hole/admin and enable DHCP from within the admin interface (this is the place you will administer the whole network from, leases, IPs, DNS related settings).
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Disable DHCP server from within the ISP router.
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Set your WAN DNS settings to point to the IP of the Pi-hole device (only) because of this:
Why should Pi-hole be my only DNS server? -
Set any port forwarding rules in your ISP router to point to your desired LAN devices.
So what this does it actually using Pi-hole's ability to be (not only a DNS level ad blocker but also) a DHCP server.
So you have two things in one: a self hosted DHCP server (on which you can set up ANY DHCP related parameter) and a blocker that self propagates to all your clients (via the self hosted DHCP server).
Your ISP router will be used to port forward and be a transition device between your LAN to your ISP.
An added bonus is that since you will be using Pi-hole as the DHCP server, you will be able to enjoy the analytics for each device that connects to the network (like sites visited, blocked, hours of activity etc.).
Have fun !