I've been having this issue for a few days now. Roughly twice a day, for 10-15 mins, my internet just suddenly goes out without any warning. Then it comes back and is totally fine again. ISP says that there's nothing going on on their end (and most of the time when I have an internet outage they do say they're having an issue)
Is the router just fucked? Are there web goblins stealing my interwebs? Is there any way to tell?
Have they sent someone out? Have you ruled out the router?
I'd try to connect directly to the modem with a device if possible and see if that device has a consistent connection or not (means you won't have wifi for a day or two unfortunately). If so, probably the router, if not then the ISP probably will need to send someone out because it's the modem, or the drop cable, or something further back down the line that they also own.
These issues were always the worst to troubleshoot because there's very little they can do for you over the phone, and not everyone is competent/thorough so sometimes if you've had one tech out there might be something they should have caught but didn't.
If you can record the specific times it goes down for a while that might also help anyone who's trying to fix it because they may be able to check the recent history for the modem's internet connectivity and if that's down when your internet is down it's narrowed down to not the router at least.
Sorry if any of this fails to help (probably will, honestly most intermittent issues callers were repeat callers IME). Honestly I had to have like three techs out a few years ago for mine, it's a hassle.
No and no, my assumption was that it probably is a problem with the router. I just don't know how to diagnose that or if there's a way to deal with it aside from just buying a new router.