I have just had a new modem installed by Telstra as part of switching to NBN service. I need to configure the new modem with the LAN IP address ranges I use on the LAN. When I go to the advanced Local LAN setup tab on the UI (192.168.0.1) and attempt to change it to the new internal numbering it says "Invalid IP address" and won't accept the change. My previous routers have all done this change no problem. And no it is not acceptable to say well you need to change your internal network numbering to the 192.168.0.xxx range.
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Did you change the DHCP address range at the same time as trying to change the LAN IP address of the modem?
There are a couple of address ranges reserved for Telstra Air and the guest network. What address range are you trying to use?
I surprised your old modem lets you us that address range and you could end up with a external site having the same address one of the devices connected to the LAN.