In this particular case, the method of infection of the router was not disclosed. However, typically, the most common methods involve an open administration port to the internet (user interface or TR-069) or through the internal interface, in case a network host has been compromised.

They often perform brute-force password attacks, and once access is obtained, they look for typical Linux administrative tools (such as bash, etc.) and proceed to compromise the router.

So I understand that a router with custom firmware can be compromised if it has a weak password and resources to maintain the infection, or of course, a vulnerability that is exploitable.

typically, these old soho routers use ancient firmware with multiple known vulns. add to that the silly practice of opening the management ports to the public internet, and you have a potential disaster.

if your older hardware is capable of running an open source, maintained OS build, then do it. added functionality plus updated protection keeps these devices useful and out of landfills.