Check the device log. There is sometimes useful information in there.
Another option for getting more useful information is crack the router open and there will either be a 4 pin header, for an unpopulated 4 pin header on the circuit board. If the header is unpopulated, its a simple matter to solder on a header. You can probably find the pinout of the header on the internet, but its not hard to figure out which pin is which too. Use a USB → TTL serial adapter with serial params of 115200,8n1.
I’ve used this method many, many times to get useful debug information from the CPE.