Wifi slowness with C6300v2

I am seeing slow Internet issues with my unit - pages loading slowly, youtube takes a while to load, instagram has a spinning wheel, etc

Background - When i first purchased the modem router combo, I had a similar issue but it was more drastic in the sense that the wifi used to constantly drop on my phone and reconnect after a couple of seconds.

I spoke to Netgear support and they tried troubleshooting. After figuring out that it might be a defective device, they sent me a replacement. Now, i am seeing the above mentioned issue with the replacement.

I have Comcast Xfinity service for my house - I rented out their Xfi gateway for a month to confirm that the issue i was seeing was related to the gateway - I was right, it was the issue with the netgear gateway

I just called Netgear support and they said that now it is a network issue and I would need to update the settings - but I am not sure what needs to be updated to work fine with the Xfinity service so I dont see this issue.

Any help is greatly appreciated!