Welcome to the community,
Could you please send me a PM (private message) with your Full Name, CID (customer identication number) and your mobile phone number, so I can take a look at your service?
Ok the information about the switches wasn’t in your post. Your setup is well out of the TPG support scope, and if the below doesn’t help I’m not sure what else TPG can do to help.
If you’re able to get an IP with all your switches disconnected, it isn’t a faulty modem. It is some kind of conflict with your switches or a device on your network.
Some tips that might help.
-Have your restarted ALL your switches and devices since getting your new modem?
-Do you have static IP address set on any of your devices not in the same range as your new modem? (Modem range will be 192.168.1.???)
-If you have restarted ALL your switches and devices I recommend you try powering down ALL your devices.
--Power your modem back on(Make sure it has sync and auth)
---Turn 1 switch on and then 1 devices at a time on that switch, and checking each devices if it is picking up DHCP
----Continue this with each switch and all devices.
Hopefully all devices will just work, but if not you will be able to work out what device is your problem device.