The article says that the "modems" affected are the Sagemcom F5380 and ActionTec T3200 which from a quick search looks like full fledged CPEs aka routers with a web interface and NAT, WIFI and all the stuff. They also write about Censys and banners so it looks like they had their web interface exposed to the Internet.

When I hear people say they use OpenWRT I assume they have their modem in bridge mode so that it doesn't even have an IP. OpenWRT would save you in that case.