C6900 expressvpn

I bought a c6900 Cable Modem Router from Costco.  It was an impulse purchase.  I'm happy so far because the speeds I get are doubled compared to the Netgear router it repaced.  The one point I wish I would have considered before the purchase was if this particular router would have a DD-Wrt firmware option, but I can't find one.  It seems that setting up a VPN was much easier with DD-WRT.  I now have a subscription for expressvpn but can't figure out how to install the expressvpn software on to this router..  Their website doesn't offer instructions about this router nor any Netgear routers.  Has anyone had any success at setting this up?