@pirateind 

Hi,

I see two possible ways to set this up.

1) Instead of routing that Ethernet cable from the "ONT/Router by ISP" to the "switch on 1st floor" route it from one of the LAN ports of the "Main router C6 v4" to the "switch on 1st floor". Then the three TP-Link EasyMesh routers would form one local network and be in the same IP range.

2) Keep the "Main router C6 v4" in "Router Mode" and set it up as an access point using the old-fashioned method that's been used for all router's that don't offer a dedicated "Access Point" mode setting. In the linked setup guide that would be "Case 2". https://www.tp-link.com/en/support/faq/1384/

Note that in this configuration the "Main router C6 v4" will complain about having no connection to the Internet. This won't affect the functioning of your network, but there are a couple of things the C6 v4 won't be able to do, like updating firmware via "Online Update" and obtaining the current date and time from a time server on the Internet. (these can be worked around by updating the firmware via a file and using a local computer as a NTP server to get the time from)

  @woozle 2nd seems to be the only viable option since It is not possible to have the wire back to main router.

If I am going to have to do the AP mode, then why not with the dedicated AP mode functionality? How will the router mode with some additional settings make a difference? I see, it said to disable DHCP and be on the same LAN ip with static ip, that happens in AP mode as well. Sorry I am not able to understand how it will help.

pirateind wrote

If I am going to have to do the AP mode, then why not with the dedicated AP mode functionality? 

I understood your original post that way that you wanted to go with AP mode from the start so that your whole network is in the 192.168.1.1-255 IP range of your ISP router, but you couldn't because on the C6 and C64 Easymesh is not available in the dedicated "Access Point Mode". (there is still hope that might change in a future firmware version)

pirateind wrote

How will the router mode with some additional settings make a difference?

In "Router Mode" the EasyMesh options are available to you and so you can create a seamless EasyMesh network between your two C6's and the one C64.

And before you start to doubt, I had a setup like that running myself, with the only difference that I had only one EasyMesh satellite instead of two. But I don't see any reason why it shouldn't work with two satellites just as well.