EDIT: Crisis averted! The problem was with the modem and not the devices connecting to it. I'm not sure why the first person I called at the ISP couldn't help me. In reality, the second person I called also didn't help, but something magically started working after talking with them a second time and rebooting the modem about 5 more times, so it turns out I don't need a new WiFi router at this time. That said, I will take these suggestions to heart, as I may be buying one anyway as a backup for when this inevitably happens again.

TL;DR: I probably need a new wifi router and want one that isn't malware and will work even if the company I bought it from goes under or stops making it.

Long version:
So today my wifi stopped working. I use an Apple Airport Extreme (the tower one that has a Time Machine backup in it). I've had it for 5 or 10 years and it's worked fine during that time, other than replacing the hard drive it backs up to. My spouse and I were sitting on the couch after lunch surfing the web on our phones, when we suddenly couldn't reach anything. The router itself appears fine. We can connect to it and see other devices that are connected to it, but for some reason, it's no longer communicating with the cable modem via the WAN port. It still backs up the computers in our house, though. I have tested the cable that was connecting it to the cable modem, and it appears fine. I can connect my computer directly to the cable modem without issue using the same cable. So my guess is that the WAN port is hosed.

However, I'm suspicious that something else is going on for 2 reasons. #1, the cable company (Spectrum) made me replace my cable modem last week. I did that, got my Airport connected to it, and after a call to tech support got it up and running. It's been working for the past week. I suspect the modem may have updated or changed configuration without me knowing it and that's the real cause here. They sent me a Wifi router with the modem, but will charge me $5.00/month if I keep it. I'd rather own the hardware. #2, I have an older Airport Express that was working the last time I used it. I replaced it with the newer model about 5 years ago so I could do backups. It fails to work in the same way. It seems like the WAN port isn't communicating with the cable modem. So, if there's some way to verify that the WAN ports on my Airports are or aren't working, I'd be interested to hear about it.

I am able to connect a wired ethernet switch to the cable modem and all devices on the switch can see the Internet just fine. I tried connecting both Airports to the switch via their WAN ports while the switch was connected to the cable modem, but that did not work. (Or at least, I couldn't connect to the internet via either Airport.)

So, on the off-chance that both my Airports have a similar failure, I need to replace them. I have gotten suggestions from others, but have been pretty unhappy with them. I have the following requirements:

• Absolutely must not store any information about me in the cloud (for example as Ubiquiti apparently does)
• Absolutely must not rely on any sort of connection to the manufacturer to work properly and must not phone home without my permission
• Preferably not a poorly made device that will die in 3 years
• Must work properly and at full speed with Apple devices
• Must not require a phone app to configure
• Must have ~3 ethernet ports so I can hardwire in my TV devices (AppleTV, TiVo, etc.)

Things I don't need, but aren't a deal-breaker if it has it:

• The ability to configure every little setting. I prefer to set it and forget it.
• Mesh networking. My house is not huge, the cable modem is in the middle of the house and my single Airport base station has good coverage of the entire house.

If you know of any device like this, please share!