Plan change with no fault of mine

Recently, I was put on Connect More Fast that goes to 500MBPS as appreciation from Xfinity. I was paying $40 per month. When I ran into an issue with my modem, I bought a new modem and a router. I called customer care to enable it  with the MAC address. After that no matter what the speed was always 90 mbps. We spent 2 hrs with no luck and a service technician was sent next day who again couldn't figure out what was going on and asked me to wait for couple of days as he said the backend systems take a while to catch up. After 3 days, nothing changed. So, I called customer care again and this time spent for 2.5 hrs. In order to troubleshoot the issue the rep told me that he would change my plan briefly to 800 mbps to make sure that it ws not the plan that was causing my low speeds. Even that change didn't do any good. I was still getting 90 mbps. Finally, I changed the ethernet cable and the speed came back. Now that we had the issue resolved, the guy said he would put me back on my original plan of connect more fast. Then he said he couldn't do it as it was $50 now. He said that he would put me on that and a loyalty rep would call me and give me $10 off. A couple of minutes later a rep called me and she said that she wouldn't give me $10 off and I could only be on connect more for $40 which gives me 300 mbps speed. I tod her that I was on 500 mbps for the same price of $40. I also told her what happened in my call where they put me on 800 mbps for testing purpose. She said that it was my fault that I agreed to change the plan. Is it my fault? Why can't xfinity just put me back on the same plan as I was and give me 500 mbps for the price of $40? She kept on arguing that she can't do that. I have been with comcast for 15 years at different addresses and this is the treatment I get. Shame on you xfinity!.