I can tell you exactly what major security patches arrived in the last two years: Samba (specifically SambaCry based on NSA's Petya), seven severe DNSmasq vulnerabilities, and obviously KRACK.
A 2.4GHz-only N router like yours is only worth about $5 used today, so if you didn't need to upgrade any features it could be cheaply replaced with one that accepts DD-WRT or LEDE, or even Tomato for ARM. Tomato for MIPs has been unmaintained for a year now, so don't get one that only accepts that.