You said that after a factory reset it works fine, but then you disable DHCP and change the IP and it stops working. What if you just change the IP a but leave DHCP on?

Now I need to figure out two more things:
1) What to do with the NAT settings. Should I disable NAT? Maybe it doesn't matter because I am not using the WAN at all.
2) How to set up the WiFi extension. Bridging the Arris's signal? Or setting up the separate WiFi capabilities of the C7. Seems like bridging is the better option because all devices would use the same SSID, right?

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Mobdro App

Yes, I leave DHCP.

Wifi extension 's DHCP is off and set it directly back to C7, so that don't my change ip.
Also NAT is on, but it don't change ip...