Remote attack surface of router in bridge mode

Does anyone know, if there is any remote attack surface at all, or small enough to be neglected, of a eurodocsis 3 type cable router put into bridge mode? Background: I need a cheap eurodocsis 3 cable modem to be used in front of an OpenWrt router. Since cable modems are not very common and relatively expensive, I was thinking about using a Fritzbox router in bridge mode to be used as the modem, which does not get OS/firmware updates anymore. I usually don’t use any devices which don’t get updates anymore, but I wonder if there is any remote attack surface in bridge mode at all or if it can be relatively safely neglected.