slow speed on G36

how are you testing speeds? 

The G36 should hit gigabit without issue. 

So I’m curious if you’re testing with a device hardwired in? Wireless?
What site are you testing on? 

Did you try xfinity’s test site?

I’m testing with help of https://www.speedtest.net/ on my mobile phone. Previously I did same testing with Xfinity modem and I saw 800mpbs + and wifi was 6e. With Arris G36  wifi is 6 and I can’t see more than 400mbs for some reason.

My laptop connected to Arris G36 via Wifi shows less than 400mbps. 
When I connected my laptop to G36 via 2.5 Ethernet cord it showed me 800mbps +

So it is evidently something wrong with my Arris G36 Wifi. 

A couple things. 

1. 6e is triband. thats 2.4/5/6ghz. Not just the 2.4/5ghz of wifi 6
2. What is your link speed on the phone and the laptop? 
3. Did you try changing the 5ghz channel the router is on?

my speed on phone and laptop (via wifi) is up to 400mbps, with XFinity modem it was 800+ on phone 

when I connect modem to laptop via ethernet cord I can see speed 900mbps +

so this is definitely wifi adapter of modem or settings.

I tried to do reset and it didn’t help 

Since you are experiencing with slow Wi-Fi speed internet connection, I suggest you to change the Wi-Fi channels and monitor it for next 24hours of time.

To change the Wi-Fi channels, find the steps in the below given link:

http://arris.force.com/consumers/articles/General_FAQs/G34-G36-Changing-the-Wi-Fi-Channel

I changed channels and it didn’t help. How can I get warranty replacement?

what wifi card is in the laptop? 

What is your link speed? 

I’m not sure about laptop but my wifi and I have Samsung s23+ and speed with xfinity modem was 900mbps+ now it is below 400mbps mostly 100-300. And I’m testing speed by sitting next to modem. in different room speed could be less than 100mbps. 

with xfinity modem I saw 6e sign next to wifi sign not it is just 6.

So you had the triband router from xfinity that had 6ghz. its another class up. 

I see what you’re saying your speeds are but I’m curious what your link speeds are. 

With  Xfinity I have 800mbps plan and I can see such speed and even slightly more (~900mbps) if I connect modem using ethernet cable. But via wifi I saw only speed less than 400mbps.