@Malo87,

Do you have your second router operating AP Mode? It is near impossible to have a stable network with two completely separate DHCP servers operating on the network. You will need to set up your C80 in AP mode, which will provide settings for AP isolation, giving you the effect of having your Smart Devices connected to a different network than your main.

Technically your setup may work with the correct configurations, however, it is highly vulnerable to changes that would bring the network down.

If you truly wanted to have two separate routers operating in their default configuration, you would need to configure VLANs, which requires much more intensive troubleshooting and more advanced hardware.

The network issue has to do with your C80 receiving during IP addresses and attempting to configure its ports for a WAN connection, when its really just an extension of the LAN. The crashing is happening because of an IP address conflict, which explains why it happens when you reboot the router, eliminating what the C80 expects to see as the modem assigning the single IP address given to your address.

The reason connected devices do not have an active internet connection, is because they are using an IP address that was given by the C80, which your main router is not expecting and does not know what to do with.

  @Malo87,

You can always use the emulators to find settings before altering your actual network: https://www.tp-link.com/us/support/download/archer-c80/#Emulators

AP isolation is found under the wireless main tab > Additional Settings.

You may still need to do some configuration of settings on the main router of your network, depending on what router you are using and how it handles the traffic.

While its a more odd solution, you may have luck setting up the guest networks of the device too, as Guest Networks normally isolate devices or have options to isolate devices in the same way you are looking for.