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Hi @mickmahony1
Welcome to the Community!
The ARC1200 will not work with your NBN FTTN service as it is a router only device. You are correct that you need a VDSL modem/router.
Since you are under the BYO plan, you have the option to purchase a third party modem/router or purchase a new one with us.
Check these articles for some information:
High-Speed NBN BYO Modem Requirements
List of Non-TPG supplied Modem/Routers for use with TPG NBN BYO Data Only Plans
BasilDV