Longtime Connection Speed Problem

Hello all, I am making this post in desperation after making call after call and tech visit after tech visit without success.

 

My problem is as follows: I pay for 600mbps however through ethernet I ONLY get max 260 mbps, and sometimes much lower, when running speedtests. This has been going on for about 4 months+. My last plan was 400mbps but one day it suddenly came down to ~250 and stayed there. Then Comcast offered to bump my plan up to 600 for free and they did, giving me a new modem and everything. Unfortunately I never in my life witnessed this speed on my wired connection. Speedtest still showed ~250mbps every time.

 

I have tried possible virtually everything there is to try. I have made about 10 calls to comcast, 4 techs have come to my house to replace my modem like 5 times. They checked the line and everything and they said it was all good, every time, so it's probably not a physical cable issue unless they were all lazy. I've probably reset my modem about 30 times this month. I've changed ethernet cables. I've used different outlets. I've used different ethernet ports. I've tested on different computers. They all gave the same results or less, never passing 300mbps, so it's not my computer's fault. In fact, really the only only thing that it could POSSIBLE be is the physical cable because that's the only thing they haven't changed out. But they won't because their equipment says the physical cable has no problems.

 

The worst thing is that the speeds are COMPLETELY PERFECT on wifi. I just got 560mbps on an ookla speedtest on my iphone XS max. The problem is just with the ethernet cable. I've just about lost all hope on fixing this problem but maybe one of you smart guys could help solve it.

 

One important thing I should mention is that every time I do a speedtest on xfinity's own speedtest, it ALWAYS gives a good speed, always above 500mbps, which is complete and utter rubbish. Forgive me for not trusting your own speedtest after witnessing what a blatant lie it is compared to the REAL connection speed. All other speedtests coincide except the xfinity one. The thing is, the other speedtests are the ones that are correct according to my actual download speed, which usually does not go above 13 megabytes/s on applications that usually download really quickly like Steam. This seemed to catch some techs by surprise because it makes no sense why the speedtests wouldn't coincide with xfinity's. If you ask me, it's xfinity themselves puffing up their numbers so you think that that's the actual speed you're getting. 

 

Modem model is XB6-A, plan speed is 600mbps download, 20mbps upload

 

PS: You community helper guys, please don't insult me by posting "general tips to improve speed" threads because I have tried everything that any of those threads could possible cover and much more.