We have Comcast high speed internet ( up to 1,000 Mbps). The modem can handle up to 680 Mbps. When it’s working, the download speeds are 300-400 range.
First. If you subscribe to their gigabit speed tier, you need to be using a DOCSIS 3.1 capable modem if you are not, you won't be able to get faster speeds.
Second. What do the modem's signal stats look like ? Try getting them here http://192.168.100.1 or here http://10.0.0.1
Please post the *Downstream Power Level*, the *Upstream Power Level*, and the *SNR* (Signal to Noise Ratio) numbers.
What is the exact model number of the Netgear modem ?
And for a test, does a computer / device hardwired directly to the router with an ethernet cable have the same problem ?
With this test, we are trying to isolate this from being either a WiFi-only problem or a problem with the general connection to the Comcast system which would of course affect both. It's a good first step of troubleshooting before we proceed any further.