Why am I still not getting my speed?

I am using xFinity xb8 modem and my problem is every since my planned changed on June 28th (ONLY my cost was to change, not service), it has been garbage and nobody can get it right. I swear if they could literally see me I bet they would be looking at me like I have two heads. My internet is way to slow for my 2 gig speed, I am pushing max approx 600 mbs and to me that is totally unacceptable when I know my system can handle the speed, and there is not a lot of devices connected at this speed and forget it when the kids get home from school, I am lucky to see 500 mbs. Also I am tapping fingers waiting for any websites to open if it even does and it is also dropping. Agent on phone did say my modem had an error, that it was not reading right.
Tech came out yesterday and just changed my modem, I asked can you please check my wires outside and he just walked around my house, looked up and said "thats all good," did not go up the pole to check for issues there and even worse he left without confirming the firmware update went thru. He showed me on his device it said N/A, so I said to him "N/A means not applicable, are you sure something is not wrong with reading or understanding your command?" his response was no its running. Hmm really, because I know when I install updates, I see "downloading & installing." Then last night my internet dropped while I was in work which took about an hour for a little bit of stability.
Today I still see no change and I am livid because I work from home, this is my livlihood and it is hurting my income yet I am paying for it.
I need someone who knows what they're doing to diagnose and fix this issue please.