Slow speed with NetGear Nightwawk 7100V AC1900

I recently purchased 7100V to replace a Arrius Surfboard modem, that starting rebooting itself after having it 3 years.  I have 400 Megabit service (Triple Play) from Comcast/Xfinity, which I was able to access with the Arrius modem.  For testing, I have used my HP EliteBook laptop, running Windows 10.  The firmware version of the 

With the 7100V, the best speed I have been able to get is about 230 MegaBits, either with hard-wired or wifi.  I have spoken to both Comcast/Xfinity and Netgear Tech support, each twice.  For testing, I have used my HP EliteBook laptop, running Windows 10, both wired and wireless.  The firmware version of the modem is V2.01.42.  Comcast ran tests against my modem and noticed a slowness in speed, but said that my service is properly configured at 400 MegaBits from their side.  Working with the Netgear Tech Support person, he had me make the following changes:

     1.  On the WAN configuration, change MTU size to 1492.

     2.  On the Wireless configuration, change the channel numbers for both 2.4 and 5 GHz.

After making these changes, I was told it could take up to 24 hours for the changes to recognized and then I should see higher speeds.  This was not the case, and after my second discussion with both companies, I felt like a ping-pong ball being bounced back and forth without a working resolution.  What I really need is a 3-way call with both vendors, but that doesn't seem like that will happen.  Comcast said that I could pay to have one of their engineer come on-site, but I would probably have to pay for that.  With the Covid pandemic, I really don't want strangers to come over.

In an effort to avoid returning the modem to the store, I would greatly appreciate any recommendations on how to resolve this speed issue.