Thanks for the replies,
everyone.
@L&LD
I still haven't been able to gain access to the router. Tried various reset methods (reset button, 30-30-30, WPS button), but it's still unreachable. No way to ping it, no SSIDs, nothing. I was able to connect my PC directly to the modem and have internet access, though, so I'm a little less desperate now.
It appears that my modem reverted to its default state (as I was able to connect to the internet without the need for the router), even though I literally never touched it during the entire process (the fiber can be very finicky and I always make sure to be extremely careful around the network devices). On Windows, ipconfig gives me a very unusual default gateway, which does not grant me access to the modem's GUI, and no other more standard addresses work. Some services are unreachable, as well. Not sure what exactly is going on.
Strangely, something similar has happened before. One day, out of the blue, that same router stopped working. I wasn't messing with it and had never used the USB ports until today, but faced the exact same issues: couldn't ping it or access its GUI in any way, or even reset it. My ISP had sent an update to their modem and I assumed that it ended up bricking my router, somehow. I simply asked them to revert the modem to its default settings and removed my router from the network, after wasting as much time as today trying to fix it by myself.
It was only recently, after leaving the router unplugged for months, that I decided to try reviving it one last time. And, to my surprise, the reset worked on the first try. No fancy methods, just plugged it to an outlet, turned it on and then pressed and held down the reset button for 5 seconds. Power LED started blinking and I was able to access the GUI and configure everything to my liking. Asked my ISP to change their modem to bridge again and all was good - that is, until the USB fiasco.
I will try leaving the router completely unplugged, like you suggested, and will follow the rest of your recommendations tomorrow, when I'll report back to you. I really appreciate your help.
@Zonkd
I will also try your suggestions when (if?) I get my router working again. Unfortunately, I'm not in the US, where the router was purchased. And even so, I bought it in November of 2014, so it's been out of warranty for a while.
If there's no way to fix it, what other routers would you recommend? I may have become slightly traumatized with Asus, after two bad experiences, but I still like their functionality. Are there any other models that are more reliable than the RT-AC87U?