Why is my R7000 slowing down my network?

I have a Nighthawk R7000. It's connected to my cable internet service provider modem via ethernet.  The provider promises me 100 Mpbs and I have been getting that or just a little more or less depending on the time of day since I started with this provider earlier this year.  Suddenly, I am now getting about 50-70 Mpbs. So I called the provider and they sent a technician out. The techinician and I discovered that when the R7000 is shut off and devices are connected wirelessly straight to the provider's modem, I get speeds 100 Mpbs, give or take a little. Re-starting back up the R7000 and connecting wirelessly to it, the speeds fall again.  We tried this numerous times with various devices and the results were the same. Which led us to believe that for some reason the R7000 is slowing the speed down. The modem supports both 2.4 and 5.0 and we tested both.  Does anyone have any idea why suddenly the speed is slower?  I have not made any changes to the modem or R7000 in the last few months.  How should I troubleshoot the R7000 to solve this mystery?