The update could have gone badly.. it happens unfortunately. Did you upgrade over wireless.. generally that is a no no and is discouraged? Although Apple does not warn about the possible bricking when done this way.
If you have tried to factory reset and had no luck then there is very little else possible. A static amber light normally indicates that the router has not managed to boot. Was it a very old one?
Although it may not have shown up hardware problems before the firmware update that does not mean something was already going amiss. Since this model has sold since 2012 it could be 5-6 years old already which is a long time for domestic routers to last.
If it is under warranty on the other hand take back to Apple and see what they can do.. or if you have applecare it could be covered for up to 3 years.. Be interesting to hear what they will do when the product is not available and people need a replacement for warranty purposes.
Outside of warranty there is the possibility of some DIY effort.. but the skills are much greater than average.. really fine soldering so you can get console access. Happy to give you pointers if you want to try.