Router manufacturers usually provide updates for their products only for a limited period of time, often leaving security holes on older devices unpatched. One possibility to provide the hardware with an up-to-date operating system (OS) again is offered by the freely available router firmware: OpenWR

OpenWrt

OpenWrt is not only the oldest open source router project, it is still actively developed and maintained today. OpenWrt offers for devices of different manufacturers beside regular current updates also innumerable extensions.

Whether a particular device is supported by OpenWrt can be looked up on OpenWrt's "Table of Hardware" page: https://openwrt.org/toh/start. The page also describes how to upload the firmware for the particular device: https://openwrt.org/docs/guide-user/installation/installation_methods/see_devicepage.

DD-WRT

DD-WRT was the first choice as an alternative router firmware for a long time and still offers firmware images for a variety of different devices. In recent years, however, development has stalled somewhat, and only beta versions have been updated since 2008. For certain older devices DD-WRT can still offer an alternative to the manufacturer firmware.