Not getting the download speed I contract on my plan

I have recently initiated a 2nd account with Xfnity on a new property. For the 2nd account, I decided to go with the 600 Mbps account, but in general I get speeds varying from 175 to 240 Mbps. I use my own equipment - a brand new Arris gateway. It's not only a new equipment, but a new account and new cable - the tech had to pass the cable from a distriution hub to my house, and the cable was later buried in my backyard. I've contacted Comcast multiple times about, and pretty much every time I call I get a different response. I've tried reseting the gateway to factory settings and having only the wired PC connected to it. Same issue still. I've replaced the gateway, based on advice from Comcast's tech support - from a DOCSIS 3.0 to a 3.1 - still the same issue. I have also looked at the signal on my internet equipement - SNR and so - all looking good according to Arris. Comcast mentioned sending a tech, but they want to charge for the visit. Would it be worth? If the signals are fine, my guess is that Comcast is gonna say the issue is on my end and not on their infrastructure and service.

Something just came to mind - as my 2 accounts are linked, is it possble the sustem is getting confused and pusshing the same speed for both accounts? on the 1st account I have 200 Mbps.

Thanks!