Setting up AirPort Extreme, share public ip address

Hi there.

I have a problem with my AirPort Extreme, which is an older model (serial: 6F7040BGVYS, Version: 7.6.3)

It's connected to my Netgear router, and it works fine. BUT when another user goes on my wifi connection, the speed goes down seriously!

I contacted my internet provider, and they told me that it was a setting in the AirPort - something about sharing public ip address..? Changing Network > Router Mode from Off (Bridge Mode) to DHCP only. When I type in the DHCP Range, it will not accept, see alert below. I tried many different, so there must be somewhere else to configure..?

Invalid Value

Sharing a range of IP addresses using DHCP Only requires manually configuring your WAN IP address. If your ISP gives you a range of static IP addresses, you should manually configure your base station with the first IP address and distribute the rest via DHCP.

How do I setup my AirPort to share public ip?

Attached photo shows the AirPort Utility, and version.

Hope someone knows about this issue 🙂

Have a nice weekend!

Best from Jeppe