The above is for an Arris router in bridge mode, to which a simple modem is similar.
You are right the MAC address override did not take effect. We’ll look into that.
As the above link also shows, there doesn’t seem to be a clear rule about what MAC address the Arris modem likes. Sometimes, the modem just doesn’t work well with the router. And the technique to fix it is more like hit-and-miss.