Thank you @raymp.
I'm not able to identify as well on where to insert the settings.
Since it is being handled by the TP-Link Engineers, let's see if what will be their action to rectify the issue with your modem.
It is in wifi router mode,
Tp link engineer just sent me an email said it is to complicated for him and he has escalated it to a senior engineer, who is now on holidays untill may 4 , waiting game now i suppose, in the mean time i will try and get a refund,
This email from Tp link
I'm sorry to tell you that this model Archer VR900 V3 doesn't support VLAN ID setup yet while using Wireless Router mode.
It's suggested to change another model or hardware version with the retailer where you bought it.
Tough luck, thanks for buying, try for a refund, go try another.
Tp links packing is misleading, it says nbn ready, what it should say is nbn ready but not HFC