Slow download speed on WiFi - Netgear C6900 Cable Modem Router

Hi , I'm posting the issue here as a last try before I return the device to Costco.

I got Netgear C6900 cable modem router as a replacement of a reneted xfinity gateway.

My internet plan is upto 400Mbps and I used to get internet speed on the xfinity gateway wifi above 360Mbps while I got a disappointment speed on the newly high speed Netgear C6900 gateway around 266Mbps on 5G and around 80Mbps on 2G using speedtest.net and xfinity speed test on my laptop, galaxy note 9 and iphone, if I'm lucky. I wired my laptop into the gateway and got download speed above 400Mbps.

Here are some other steps I've done:

1- Contacted xfinity Support to complain about the speed sloweness issue, after troublshooting the issue they said nothing can do from their end and recommended to contact Netgear Support.

2- I contacted Netgear Tech. Support, opened a ticket, and did all the fixes that they recommend (changed channelsfrequency, the 24 Downstream Bonded Channels are all locked, device restart and gateway hard reset) and that didn't help. In the end told me I should wait about 48 hours and contact them again if didn't get resolved.

3- After 55 hours I contacted Netgear support again and went over the troubleshoot steps once more and that didn't help as well, then siad I need to contact xfinity Support to do a firmware re-provison.

4- I contacted xfinity support and asked them to do firmware re-provision and that didn't help also.

Any advice on fixing this issue would be great.