Using Pi-Hole with 2 routers

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.

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

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).

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

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

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