Own modem and router

I was told today by an Xfinity (online help, level one support) guy that I need the Xfinity modem for the Flex 4k to work. Is this true, as I just returned my device? That's after spending two days on the phone with them and talking to multiple techs. Or is this a case that Xfinity people have NO idea what they are doing or what's going on with their own products?

 Does anyone uses this with your own modem&router? I had mine connected via Ethernet also. Thanks.