Agree with c6em
Thing is, if an ISP has a standard modem, then they can stand by its performance. After all, if a user picks a horrid modem and then reports faults, that's not fair on the ISP. Maybe recommending a modem is the way to go. You can't force users to use one, well maybe you can, but you shouldn't be trying to.
My second point. Must surely save money if customer service have a standard router whose UI they know so they can talk the user round it efficiently. Simply can't expect customer service to know what is going on with every known router out there. Maybe the way to go here is to say, “use our standard modem if you have a fault, otherwise fix the problem yourself.”