CM500 Modem gets half speed when connected to router but full speed when connected to PC

I recently purchased a used CM500 to replace a Mediacom-provided unit that we rented.  We subscribe to 100/10 internet service and have always gotten full speeds.

The CM500 was provisioned and I tested it connected directly to my PC and got the full 100/10 that we pay for.  However, upon connecting it to my existing router, the speed drops to 50/10 across all wired and wireless devices including the same PC.  I double checked all the router settings and even did a factory reset.  There is no bandwidth or QoS enabled.

I then went back to our Mediacom-rented modem was able to get the full 100/10 through this same router.  This leads me to believe that it's the modem that's causing the issue, but I have no idea why the router would kill half the bandwidth but only when using a router.

There have been a few people on the internet who reported similar issues with other brands/products and the solution seemed to always be get a new modem. There are very few settings on the CM500 so there's really nothing I can play to get it working properly.  Was wondering if anyone had any knowledge on why this would be the case?