I had this router laying around and wanted to put it to good use. It is a CISCO model EPR2320R2. I couldn't find this model in the hardware list but I've seen there are other CISCO routers in the hardware table so maybe it can actually support some release?
The chip inside is a BCM3349, again I've seen other BCMs in the index of releases but not this one, but maybe it is compatible...
That already gives you an answer, it's unknown, meaning it's unsupported at present (if it were supported, enough would be known to list it in the ToH).
As a cable modem, chances are slim already (the cable modem part will never be supportable, nor do drivers for broadcom phone features or broadcom wireless exist, so at best you could make it a wired-only ethernet router, which isn't really all that interesting for a device with a single ethernet port…). Apart from this, Cisco is one of the few vendors who like to add genuinely custom hardware, meaning that their devices are often much harder to support than the more generic hardware manufactured by other vendors, who tend to follow the chipset designer's reference implementations. So be aware of the implications of Broadcom hardware (and their lack of drivers for anything but naked SOC and ethernet/ switch), Cisco and 'cable modem' in particular.
Even in the best of all cases (which yours certainly isn't, meaning non-cable, non-Broadcom, non-Cisco - just a new device for a well-established, well supported (all drivers basically exist) SOC), you'd be looking at many days to a few weeks of basic documentation- and development work to get a new device supported - the less is given, the more work you'd have to put into it (we're quickly talking about months and years).
Realistically speaking, you'd be much better off by selling the existing Cisco device as-is and then looking for a well-supported (by OpenWrt) used device on the second hand market. With patience and persistence (and if you inform yourself about the support state for OpenWrt), you can get very capable hardware for 10-20 bucks already - and guesstimating your location to North America, e.g. https://openwrt.org/inbox/toh/askey/rt4230w_rev6 often sells for around 20 USD over there (giving you (previous generation, but-) pretty high-end hardware for a song), obviously there are other alternatives and different options in different countries, with https://openwrt.org/toh/linksys/e8450 being a current favourite often selling for ~50 USD new.