Prepaid Cable Modem

I already have Fiber internet from other internet provider and just open Xfinity Prepaid account as secondary option to use when primary fails.  I own an advanced ASUS router that supports dual WAN and can switch automatically if one of the WAN connection fails.  I have setup several rules and policies on ASUS router to meet my needs.  Obviously, I am not interested to use WiFi feature of Xfinity Prepaid cable modem.

The speed of Xfinity Prepaid service is same as advertised but I have problem with Xfinity Prepaid policy to use specific cable modem that comes with Prepaid kit.  I want to use another cable modem that is compatible and used with Xfinity postpaid service in the past.  The Arris TG1682P cable modem that comes with Prepaid kit has following issues that cannot be avoided.

1. It keeps broadcasting radio signals of 2G and 5G bands, even when I disable both bands from its console.  It just makes APN name invisible.  I do not want unnecessary radiation in the house.

2. It is huge in size and generates lot of heat that is not good for other devices that I keep in special area in the house to protect them from kids.

3. It was found buggy when used in "Bridge" mode.  My ASUS router gets WAN IP from cable modem only when this cable modem is power recycled.

Kindly allow me to use my own modem that worked in past without above issues.