It only needs to be setup initially as below, after this the power up sequence does not matter.
1 - Set Hub 3 in modem mode and switch it off.
2 - Setup the Archer in DHCP/Automatic IP mode and let it settle.
3 - Connect a Cat 6a cable between the Archer's WAN port and any port on the Hub 3.
4 - Switch on the Hub 3.
You should now get a WAN IP to the Archer. That's it done.
As for the wifi extender it may work with the Archer, or you may find you don't need it anymore.
I'm a Very Insightful Person, I'm here to share knowledge, I don't work for Virgin Media. Learn more
Have I helped? Click Mark as Helpful Answer or use Kudos to say thanks