Using Pi-Hole with 2 routers

General
1

Last week I changed from Internet Provider. I received a new modem/router from my new provider. This is a Genexis Platinum 7840.
I need to use this Genexis router, because it has a integrated fiber cable. This cable plugs into the fiber main connection of my home.

My previous provider uses also a Genexis modem, but this was only a modem. They also send a Fritzbox 7490 router.
In this setup, Pi-Hole works fine. But after the provider change, Pi-Hole won't work.

I setup the Genexis as a DHCP server, and the Fritzbox is setup as a access-point. I use the same IP range as before.
My Fritzbox is placed outside the DHCP pool. The WiFi is also setup at the Fritzbox. At the Genexis I switched off WiFi.
I use as Raspberry Pi Zero W to run Pi-Hole. I also run PiVPN, but I need to reconfigure PiVPN, so I don't think that is does matter at the moment.

When I power off and power on my Genexis and Fritzbox, Pi-Hole will work for a few minutes (to see in graph in the user interface, and websites don't show adds), but after that Pi-hole stopped working.
I do have internet connection, and the DNS is also pointing to to the Pi-Hole IP address.

So, can anyone tells me what is going wrong here?
Needs the Raspberry use the WiFi of the standard gateway? The Genexis in this case.
I also reconfigured Pi-Hole, but unfortunately without success.

Thanks.

2

Call your ISP and ask them to put their hardware into Bridge mode. this will allow you to use your router as you did previously

3

I did. But my ISP says that they can't or want put the modem in Bridge mode.
If I understand you is the problem that I use 2 routers?
Preferably I use only the Fritzbox. At my previous ISP I never had problems with the Fritzbox.
On another forum I read a lot about the Genexis 7840. Many people are not happy with it.
There is another solution, to use a own fiber router (SFT to Ethernet I means).

4

i would suggest putting the IP of YOUR router in the DMZ of the ISP provided router. It's not bridge mode but it is the best you may get

5

I'm not sure about DMZ. If I read at the Internet, is DMZ not really safe to use.

6

the reason for putting the furthest downstream router into the next routers DMZ is to prevent Double NAT also you would still have the HW firewall in the second router