Hello, I wanted to add a router behind that of my ISP, but I had several questions,
- Do I have to connect Pi-hole directly to my ISP's router or to the other?
- Is it possible to have a 10.X.X.X network?
You can do both, it's just a matter of settings and network topology.
IF you connect other devices to the ISP router, it would be preferred to connect the Pi-hole to the ISP router too.
If you connect all device to your other router then it would be preferred for Pi-hole to be connected to that second router. It helps with management 
You can setup any type of private IP range within your LAN. As long a everything else matches and belongs to that class, you should be set
Thank you for your help, so I will have my equipment on the router, the problem is that my ISP's router has an IP in 192.168.1.1, so pihole is in 192.168.1.X, is it that by putting the router openwrt in ip 10.10.10.1 they can communicate? Or do I need to change the range of the Pihole DHCP server, and will my ISP router still be accessible?
Yes.
So from a device connected to the OpenWRT (10.10.10.x) you will be able to access 192.168.1.X .
Anything (including Pi-hole) will be accessible however, from a reporting standpoint of view, Pi-hole will see all the clients connected to OpenWRT, as 10.10.10.1.
So what do you think of :
ISP Router : 192.168.1.1
OpenWRT : 10.10.10.1
Pi-Hole : 10.10.10.10
Pi-Hole DHCP Server Ranger : 10.10.10.100 to 10.10.10.250
And as i understand, all DHCP clients (of pi) will be able to communicate with another subnet, even if i connect a switch on my ISP router
That works.
All clients with be able to reach the ISP router and also use Pi-hole as the DNS (due to the fact that Pi-hole is the DHCP server).
Remember to disable the DHCP in OpenWRT
Sure ! And on the pi if i understand correctly, i put the ip of the openwrt on the gateway, and not the isp router
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