Did you change the channel for the Archer C9? Change the channel for 2.4GHz wifi to 1-11. If still not working, can contact the extender's support and see if there is any suggestion.

Normally need to type in the APN correctly in the interface to get internet. Go to Network > 3G/4G, click Advanced, check " Set the Dial Number, APN, Username and Password manually" then type them...

Refer to this link and try firewall on the modem: http://www.tp-link.com.au/faq-467.html Allow the 3 network addresses for your phone. Allow all target for all of your other LAN hosts. Choose deny...

If the devices are in same IP range(eg. they are all 192.168.0.X), they can access each other. Make sure your PC is obtaining IP address automatically and have same IP range with other devices.

If you need to use VOIP function, the device should be a NAT device, Access point mode is not a NAT device, you can set it up as wireless router and connect your original router to the LAN/WAN port,...

Not sure why, but my router need about 1 minute to start up, I thought it is fast. But seems if there are more function on the router, it need more time to get ready.:confused: