Internet issues and a bad customer experience

I am submitting this forum since I have been having consistent issues with my internet speed, everytime I reset the modem the internet speed for my devices will go back up to the 500 mbps speed andafter 5-10 minutes I have issues of the speeds going back down to 20 - 30 mbps, this has consistently happened on all my devices and at times I have had the download speeds be 3 mbps.  

This is my ultimate issue when on the call with the Cox representative and letting them know I have updated the COAX cable made sure to reset, and all the common troubleshooting steps they said they would do a ping test, they said after the ping tests, it shows no packets were lost, but still I have internet download speeds. They began to get very terse with me on the phone and changing their tone being very argumentative and insisting that it is my device that has the issue, I let him know that since this is happening consistently in every device I own, PC, tv, phone and even my brand new laptop that there was no way that it could be a device issue, especially when all devices are experiencing the same thing of a modem reset increasing speeds for 10 minutes only to go back down to bare minimum speeds.
I requested for a Tech to come over and check my issues but after talking they kept interupting me saying that the Tech would not be of any help and its my own devices with these issues, they then tried to upsell me with Cox complete care but after wanting confirmation if  I could get a Tech with Cox complete care they said I was being difficult and that I kept asking the same questions when I simply wanted clarification, I was told cox care does not do Tech and felt frustrated that they would try to upsell me something that would not resolve my issue.