Costco CAX30 Router

I'm not sure why that would be, but downgrading the firmware shouldn't be the cause for that since that's the same firmware that would have been on your router a few months ago.

The reason you can't go back to the 2.1.3.10 firmware is because it was pulled and the reason for that was that it was causing issues, so it's not a mistake to revert to an older firmware. I had issues with intermittent 5G until I rolled back to 1.4.11.2 and have been getting roughly 500Mbps on it with no interruptions ever since. The only other thing that I can suggest is doing a factory reset on the router, or at least try restarting it again. There may also be settings you want to check to see if anything has changed, such as the SSID channel being used. Netgear has an app called NETGEAR WiFi Analytics. If you select the fourth icon from the left on the bottom (it looks like a radiating circle) it will tell you which channels are being used by other networks around you. If you live near many others, like in an apartment) there's a chance that too many other users on the same channel could affect your internet, so you can try a different channel.