1. If you want to use the VOIP phone service bundled with TPG, the TPG router has to connect to the NBN box. If you don't want to use the VOIP service, you have a choice of which to connect to the NBN box. If you connect the Deco to the NBN box, its wifi might be sufficient to not use the TPG router.
2. To use the Deco in place of TPG router, the connection type is PPPoE which requires your username with @TPG.com.au and your password (not the Community ones). In Advanced Features, Enable VLAN mode, Internet VLAN ID=2, priority=0. You can compare the settings between TPG router and Deco.
If you need the VOIP service, the Deco setup is different. There are several posts about connecting a second router and I'm not clear on what works.