I'm about ready to give up on ever having the speed I pay for

I have probably spent 14 hours the past week working w/ several Cox reps trying to resolve an issue w/ my internet connection. I have a ARRIS SURFboard AC1600 DOCSIS 3.0 Cable Modem Router (SBG6700AC) I bought it a year ago to connect with Cox after I moved in to my apartment. I had a 50mbps package and always got speeds of 25mbps-30mbps download but I wanted closer to 40-45mbps download speed to get what I pay for. After several hours of talking with reps I agreed to try the 100mbps package to see if it would make a difference in my internet speed. So now I get 40mbps download after my speed increased. My upload speed is always fine, my last package I paid for 5mbps upload and always got 5mbps upload, This new package I get 10mbps upload and again always get 10mbps upload. I don't really see why I only get 40mbps download when I just upgraded to the 100mbps package. Its not worth the extra money compared to the last package I purchased. I'd rather just have my last 50mbps package back and expect the 40mbps download speed like its intended. I kept getting blamed by reps for buying a router and not using one Cox provides. I checked Cox's approved list first before buying this modem/router combo and the Cox rep that set my internet up was even the one that recommended this one to buy to me when I moved in. I checked with the Arris customer support to figure out how to run diagnostics and check if everything is working properly with the equipment. After working with the modem/router company as the equipment was still under warranty it was concluded that nothing is wrong with the modem/router and its working perfectly fine. I feel like I'm being scammed :( The reps that talk to me are nice but they are slowly starting to lose their patience I think as I have to explain all of this every time I talk to someone new. I keep reading more and more about all the horror stories to get a speed issue resolved just ending with the speed never being fixed or it working properly for a day and then going back to the way it is. Is this just the usual type of speed you should be expecting from these sorts of packages? I don't use the wifi feature either on my modem/router and have it plugged directly into my PC but no matter what I do I am not getting anywhere close to what I pay for. I don't know what others people experience is but is there really any difference from using a rented router/modem from Cox? Just seems like a scam or like my speed is being slowed down on purpose to like pressure me into getting one of the ones Cox offers when the one I bought should be perfectly fine as Cox even has it approved on their list.