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@Tinhnghia . So, you've connected the Orbi router to the Archer so you can use the Orbi mesh wifi. Do you make any change to the Orbi router when you connect it to the Archer?
I would expect that the Orbi router would not work if it still has PPPoE and VLAN setting.
Yes I had to choose not required log in ( PPPoE) and untick enable the vlan then orbi will provide internet as normal. I wish to use orbi to replace the archer 1600 but unsuccessful and tpg can not help me, they said I need to contact Netgear, I did but Netgear also ask me to contact TPG .
@Tinhnghia . Regarding password, it should be same as Archer. It might be generic: noPassword , or the one you use for My Account.
You can try username with and without @tpg.com.au .
There is also Orbi Operation mode. When connecting to Archer, Orbi is Access Point. When connecting to FTTC box, it needs to be Wireless Router mode.
Still need to check with TPG whether the Orbi is authenticating.