Below are a few alternative operating systems, that can be used on routers, Wi-Fi access points etc.
OpenWrt is an operating system (in particular, an embedded operating system) based on the Linux kernel, primarily used on embedded devices to route network traffic. The main components are the Linux kernel, util-linux, uClibc, and BusyBox. All components have been optimized for size, to be small enough for fitting into the limited storage and memory available in home routers.
You can consult OpenWrt's table of hardware to check if your device is supported.
pfSense is an open source firewall/router computer software distribution based on FreeBSD. It is installed on a computer to make a dedicated firewall/router for a network and is noted for its reliability and offering features often only found in expensive commercial firewalls. pfSense is commonly deployed as a perimeter firewall, router, wireless access point, DHCP server, DNS server, and VPN endpoint.