Docsis 3.1 SB8200- getting 300mbps speed only. Have gigablast plan

I have a RT-AC5300. I did connect my pc directly to the modem but it didnt obtain an ip address.

You have to power cycle the modem each time you change what it is plugged into(PC or router). If connecting to the modem is fast but through the router is limited to 300Mbps, the first thing I would do is check QoS and NAT acceleration. Looks like the router itself bottlenecks around 791Mbps as per here.

What WMO said. You have to powercycle the modem, so it see's the new system, and the server at cox knows to issue a different IP address. 

I'll try that when I get home later today and post the results.

I sure appreciate what you and Darkatt do here. I remember a conversation long ago with a Cox rep on the phone. I was frustrated already because the previous person rebooted my modem remotely, even after I asked them not to, which took down the phone, so I had to get back in queue. The person said to connect directly to the modem and see if everything worked better. Nobody had mentioned power cycling before, so I didn't think the direct connection worked, plus it meant I'd lose the phone again. I vented a bit.  I learned the details later, but not from Cox. Always felt a little bad once I learned the right way to do it.

Great! Glad to hear it's all 100%. Believe it or not, I have found that 9 out of 10 times, or MORE even, the router is causing an issue. Don't know why, but slow downs, WiFi issues, strange drops, and many times, a factory reset squares it away. 

A factory reset is good troubleshooting but if the issue comes back you are going to have to do some deep diving into why the problem happened. Did you change any settings from default when you first installed the router? Did the speed go back down when a certain device reconnected to the network?