You can configure FQ_Codel to match on different parts of of the packet. Default matching "tuple" for flows includes source/dest port along with source/dest IP. If you set this to source IP only it should fairly distribute bandwidth based on LAN IP alone. If cake is a normal QDisc you might be able to enable flow-per-ip without adding any code. For an example with FQ CODEL see:

$TC filter add dev $IF_WAN parent 11: handle 11 protocol all flow hash keys nfct-src divisor 1024

At https://openwrt.org/docs/guide-user/network/traffic-shaping/...

Unfortunately every modem I've seen, including "business" models, doesn't use CODEL.