I don’t think Cat 7E is a real Ethernet cable specification. You probably have a CAT 5E cable, and in most cases, you probably don’t need to upgrade. See below for details:
Choosing an Ethernet cable spec is dependent on the maximum bandwidth you need and the maximum bandwidth supported by the ports you’re connecting the cable to. In the case of connecting your modem to your router, the limiting factor is—for most people anyway—going to be the upload/download speed from your ISP.
Without getting too deep into the weeds, here is a general rule of thumb you can follow:
Is it a short run (less than 55 meters)? Buy CAT6 cable.
Is it a long run (e.g., through the walls of your house or between buildings)? Buy CAT6A cable.
From my experience, these days, CAT6 cable is just as cheap as CAT5E cable (if not cheaper).
Additionally, if you already have a CAT5E cable laying around and your download/upload speeds don’t exceed 1 Gbps, there’s no need to upgrade the cable.