I had one of the weirder computer problems I've encountered over the past two days. It all started when my wife couldn't connect to Diablo IV. She would just get a message that the network servers couldn't be reached and the game would close. So I figured the Diablo servers were just having some issues and it would fix itself. However, the friend she was playing didn't have any problems and I couldn't find any mention of an outage online. Even stranger, her friend did see her account come online for a few seconds before being disconnected, so she must have been making connection to a server. She eventually fixed the problem by going through a VPN.
Later in the evening, I noticed I couldn't load Google pages any more. After a bit of testing, it seemed I was able to connect to some websites, but not others, but I couldn't really find a pattern into which websites worked and which didn't. One of the strangest things was that I could go to Reddit, but it would only load 4 posts. Refreshing the website would give me 4 different posts though, so it wasn't a caching issue. We had already reset the router, so I figured it must be a problem with our ISP which would probably resolve itself in not too long.
The next day I worked at my office all day and my wife was able to use the VPN to connect without any issues, so I didn't think about it until the evening. When I tried to open the YouTube app on my phone, it took a while to load the overview of videos, but eventually it did. When I then opened a video, it just kept buffering for a while, but eventually did load the comments and the title of an ad, but the ad itself didn't load. So obviously it was making a connection, but for some reason it was really struggling. I was too tired to bother trying to fix it then though, so I just switched to my phone's data instead.
I was honestly a bit worried we had gotten some malware that was flooding our internet with traffic, because it seemed that most of the things that did still load contained mostly text, like this forum. Since I was working from home today and I didn't want to be stuck using my phone as a hotspot, I decided to get to the bottom of this. I checked the Network tab in the browser console window for a couple of websites, but that didn't really help. Then I decided to reset our router to factory settings, but when I logged into the configuration page I couldn't find a button for it anywhere. However, while I was going through every page looking for that button, I noticed that the primary DNS was pointing to a strange IP address. At which point I remembered I set up Pi Hole a while ago, but had eventually abandoned it. After changing the primary DNS to 8.8.8.8 everything worked again.
I did have a secondary DNS set to 1.1.1.1, which I suppose was how we had been able to connect to everything after the Pi Hole had been shut down, but it seems it's experiencing some issues and we had nothing else to fall back on.