@B-Rad-Today 

The 3 Deco are identical, you can use any as the main.
The Deco will choose the best method to connect, either in daisy chain or in star.

Are you using cable or wifi when you observe only getting 400Mbps out of the main?
The M4 should support up to 1Gbps.

  @yves_b thanks. I had Cat5e cables and put Cat6 on from modem to main deco. Then tested over WiFi and got 381Mbps. My laptop doesn't have a Ethernet port to test it wired. When I increased from 500Mbps service to 1Gbps there was no increase on the WiFi speed. Spectrum came out the other day, tested the lines and the modem, said it was all good. So I don't know what the issue could be. 

  @B-Rad-Today 

I believe 381Mbps over wifi (5) is a very satisfying number, and exceeds the throughput required by most usages.

If you want to achieve more, you could use a USB to Ethernet adapter on your laptop, or migrate to wifi 6 (new Deco, perhaps new laptop).

  @yves_b thank you. I'm not worried about the laptop so much, all the chat support kept telling me to use it to test the wired speed. 
 

I changed the Ethernet cable to Cat6 from modem to router, do significant increase in WiFi speed out of the deco. 381-427 were the speeds I was seeing testing WiFi speed on my iphone13 pro standing next to the main deco. I'm paying for 1Gbps from Spectrum. When I increased the speed from them there wasn't an increase in WiFi speed from the deco. To me it sounds like the deco is maxed out on how much it WiFi it will put out. Tp-Link support says the deco M4 I have should handle 1Gbps. But only getting up to 427Mbps WiFi. Just doesn't seem right to me.