@DocDam Try to hard reset the router and re-configure from scratch. After FW update it helps in most of the cases.

I would use option 1, but with C4000 as a main router and C1900 as an access point, because C4000 has more features. As an AP/Mesh system you can use a DECO. With such system you can even get rid of...

@DocDam Look at the following screenshot: If these are checked - un-check them for both 2.4 & 5 GHz bands and click Save. Then you'll be able to communicate between these devices.

@Imran0321 You can use the WDS function to set the AX600 as a wireless repeater. You can change 5GHz channels from the router menu. You can browse through AX6000 emulator to take a peek at the...

@Gavin2020 Well, the other thing I could suggest is to switch functions of C80 and C60 devices - C60 as a main router and C80 as an AP. This way you could isolate the issue if it's with the DHCP...

By FW I meant firmware. So if it's EU version go ahead with the link I posted earlier.