If you already have reset the device after FW upgrade (by the button on the back), re-configured from scratch and it still doesn't connect, then that won't help either. In that case follow Kevin's...

@panoptic Sorry to hear that, and yes, you could do that in the future, according to your needs. And you still can use the LAG of AX6000 in your LAN if you ever need it.

Yeah, it's a little bit tricky for the first time.

@MonthOLDpickle Here's a screenshot from C1200 v3:

Most probably yes.

@OceanWanderer Where did you buy this device from ? The FW version you stated is newer than the official one in the support portal for your region. I believe that's the reason for having the error...