Model R7800. What does a solid amber lighr mean?

Depends on which indicator LED you are asking about.  Different indicators, different meanings.

If you go to the top of this page under Downloads, feed in your R7800 model number, it will take you to a page where you can download the user manual.  Starting on page 163 of the manual, there are several pages of information about troubleshooting your router using the LED indicators.