I'm having strange network problems that I cannot explain. It started around 2 months ago.

I currently live in a house where everyone connects to the network via WiFi. I had been nagging my ISP over this problem ever since it started.

The problem is that, without any error, the internet will simply stop working on all devices. Even my localhost servers that I use while developing apps stop working for the duration of the outage (seconds).

The problem got so annoying, that I decided to switch to another ISP. The other ISP installed entirely new hardware, a fiber optic line, new wiring, etc.

The problem is still here, and I'm starting to believe that maybe one of my housemates has a device that is causing an error on the network (DHCP? I have no idea).

The problem seems to remain even when I am the only one home, which leads me to believe that it may be a chrome cast device or a smart tv.

Any help on how to diagnose and fix this?

With even localhost stopping from working this sounds like a problem with your computer. Unless these services depend on some internet-service, then it might be something else. Perhaps some piece of malware, making you part of a botnet. I guess a DDoS might explain the internet going down for all other devices as well.

It depends. I’ve seen localhost externally routed before on both Windows and Linux. It should use a loopback device but it doesn’t always work that way.

It sounds like interference. WiFi is designed to cut out like that when there are competing signals. Good citizen devices are supposed to share the unregulated spectrum with WiFi but often don’t. Sounds like you or your neighbors have one of those devices. Microwaves, cordless phones, and baby monitors are the usual suspects.

To spring board off of Jon'C, you need to use a WiFi analyzer like Acrylic. You can download a free trial here:
https://www.acrylicwifi.com/en/downloads-free-license-wifi-wireless-network-software-tools/download-wifi-analyzer-windows/

It will show you how the different WiFi connection channels are overlapping on the frequency bands. You can then determine which channel that you need to change your WiFi router to. Generally, they should be set to auto and the different devices should figure out which is the best band, but if it's crowded with too many WiFi connections, it may not be able to make that determination and it ends up defaulting to channel 3, 6, or 11. Also keep in mind that the higher the frequency the better the speed, but has a weaker wall penetration (signal strength will drop off more through walls). Lower the frequency the slower the speed, but has better wall penetration (signal strength won't drop off as much through walls).

Are you using the same router for the new internet service as for the old internet service? I bought a brand new netgear router from costco that had a firmware bug that would drop internet on all wireless clients periodically. The wired ones would keep working but all wireless ones would start timing out and eventually just not have internet. They could talk to other wireless clients and they could access local network resources, but they'd time out on any non-local connection. Netgear support was worthless and I found out after the first 90 days you have to pay for phone/email support (even when the thing is still in warranty). I just returned the darn thing to costco but apparently about 9 months later they did release a firmware update that fixed the issue.

Maybe you just have a busted router.