No internet access, intermittent problem on all devices

This started with one PC, but now all devices are experiencing the same issue. PC devices can connect to the local network, but no internet access. Phones just constantly connect and reconnect every second. On some devices, I can connect if I assign another DNS server manually, but that doesn't work on others. The problem is intermittent. I have tried the following multiple times:

- Powered off/on the router and cable modem

- Checked connections

- Completely reset the router to original factory settings, performed all updates and changed network name/password

- Completely reloaded my PC to factory settings (this was when the issue was limited to my PC)
- Completely reset network settings on my phone

At this point, I'm wondering if I should just buy a new router and/or cable modem, but after reading about other people that had this same issue, that hasn't worked so I don't want to spend a bunch of money for nothing. My current equipment is as follows:

Cable modem: Netgear CM500
Router: Netgear Nighthawk whole-home mesh system, MR70