I assume that you have an ISP modem as well as a router? The Asus RT-N66U/N900 doesn't seem to have a built-in modem, but I haven't checked much. Whether routers have, and can use, a built-in modem depends a lot on the technolgy your internet connection uses. I'm in Australia so I don't know what may be required at your location.
Anyway, the point is that if you have a separate ISP modem, then the router attached to it was never the problem. Also, as the modem simply outputs an ethernet connection, you can connect any router you want to use, because the router is inside your home network, and not part of the ISP network. If you have a router with built-in modem, almost all of those can be put into "Bridge Mode", which means that they act only as a modem, and effectively you now have a separate modem, so can add any router you want. Of course you would still be renting the ISP device, but maybe you can buy your own modem as well? In Australia only ISP modems can be used, and it is those that are managed by the ISP, not the router. Although the router does typically have cutdown functionality compared to the retail version of the same device.
The exception to the above is when a single modem/router is also providing other services, such as VoIP telephony, where putting the device into "Bridge Mode" may prevent the extra features from working. For example, I currently have a HFC (Hybrid Fibre & Cable) internet connection with a separate modem (Good), and an ISP router that provides VoIP telephony (not so good). I can't change the router without losing the VoIP telephony, or buying equipment that also provides that functionality. But it is a pretty good router (Netgear) and also provides failover to Mobile Broadband if the HFC connection has problems, which is very handy.
The Asus RT-N66U/N900 is a pretty good router, according to most reports. Some reports weren't favourable, but I think all issues have been fixed by firmware upgrades.
If I were you I would be looking at putting the Asus RT-N66U/N900 back into service, and have the ISP justify why you shouldn't, if they object.
Maybe share more about your ISP and internet connection type, and you could get better advice, with local knowledge.