OK, thanks for confirming that. This tells you that the AirPort Extreme is configured correctly and that it is acting as a pass through device to provide IP addresses to all of the clients on your network from the Telsta modem/router. In other words,the AirPort Extreme is not involved at all with issuing IP addresses on your network, it simply passes through the information that it receives from the Telstra device.
It is up to the DHCP Server in the Telstra modem/router to provide correct IP addresses for devices on the network, and based on your report, it is having some difficulty providing this service correctly.
Try a complete network power cycle to see if a fresh start will help the Telstra modem/router keep better track of devices on the network. To do that, power off all of the devices on the network.....everything, computers, mobile devices, printers, etc....in any order that you want.
Wait a minute, then power up the Telstra modem/router for a few minutes. Then, power up the next device connected to the Telstra device and let it run for a full minute. Keep starting devices one at a time the same way about a minute apart until the network is back up again.
If the Telstra modem/router is still having trouble with IP address assignments, it may likely be defective, and should be replaced.
If you want to continue to use the same Telstra device, then check with your provider to see if it might be possible to configure the Telstra to provide a fixed, static IP address for each network device. In other words, every device on your network will always receive the same fixed IP address time after time.
It is quite a bit of work to set this type of feature up, but it might help the Telstra keep things straight on the network.