So the situation is: 

I want to set up a main router, which is CISCO linksys. I want the CISCO to have a DHCP enabled. I have two Archer C6 routers, that were used as Access points until now.

BUT

While archer C6 routers are in the AP mode, L2TP passthrough is somehow disabled, or not working, so im in a siutuation, where I can only connect to my jobs servers using the main CISCO Wi-Fi, or wired connection.

When I enable C6 router mode, and connect cable from CISCO, through a switch to WAN port(Cable working properly, connected to a switch, when I connect it directly to PC, it works fine) Router says that WAN port is not connected (i set the mode to dynamic IP, as it should be). There is no apparent way to enable any other port, to work as intended (connect router to the internet, get IP address and work as AP). The situation repeats itself while using the second router aswell.

So my question is:

How to enable router mode in the routers, and use them as APs, with enabled or working L2TP passthrough, giving the fact, I cant configure it in a way given above.

Thanks in advance for your help.