@StayAway
Welcome to the community.
Is your NBN service still over a DSL connection?
Deco X20-4G could be used for Ethernet connection with 3/4G backup when it is set to be the main Deco. So It will work if the NBN provides an Ethernet connection.
How to set up 3G/4G or 4G/5G Backup on Deco router
I have seen feedback before that A user has both Deco X20-DSL and Deco X20-4G in the same Mesh network. He wanted to achieve:
Deco X20-DSL is serving the DSL service as the main connection and when DSL failed, It would switch to Deco X20-4G which starts serving the 4G mobile service.
It is a good idea but Deco does not support it so far. Currently, both the main and backup connections need to be from the same Deco unit.
Thank you very much.