simply replacing my netgear router with a asus BD4

I replaced my netgear router with a Asus BD4 router.  The cable modem is still a Motorola SURFboard SB6120. After doing a reset of the SB6120 through the Xfinity app, everything is working fine.  The problem is that after some time, the connection is dropped.  This has happened 3 times so far today.  Each time I reset the SB6120 it works again but this is unsustainable.  Any thoughts?