I have an AVM 7590 Fritzbox.
I have now bought the GL-MT6000. I want to set up a new network via WLAN, but I want to use the same IP address as in the Fritzbox. I want all devices in the network to be accessible. Is this only possible with cable WAN?
Any chance of a diagram of what you are aiming to do, is the MT6000 replacing the Fritzbox?
The Fritzbox should remain and the IP range should also remain. I want the GL router as an extension but via WLAN in the same network as the Fritzbox.
You can NOT have multiple IPs with the same numbers on the SAME network, they have to be unique. Otherwise where is network traffic meant to go to. It's the same if you and your neighbour live in the same street and both have the same number on the front of your houses, where is Mr. Postie meant to deliver your letters too?
With that in mind, best supply a diagram of what you want to achieve.
@Confucius is right, you need to make sure that there are no duplicates of the network addresses.
Easiest way might be by using the Flint 2 in WDS / Repeater mode.
However, keeping the Fritz!Box and additional routers is mostly a bad decision, tbh.
Other option you could make access point mode for GL router.
That's basically how I'm set-up.
Give the GL-INET600 192.168.178.2, then you know the two routers are 1 & 2, then number the others as you wish.
If you are using DHCP on both make sure they give out ranges that do not overlap and do not include any static IP's you have. You might lose DHCP on the GL-INET600 anyway when you change it's mode.
After you have done all the setting-up. Run the GL-INET600 in repeater mode, pass through mode, bridge mode or access point mode or similar. I don't know the GL-INET600 so can't give you advise on it.