It looks like ASUS is being proactive to their registered users of several router models.
Here is a copy of the email that was sent out today.
ASUS Router Security Update
New firmware available for RT-AC5300 / RT-AC88U / RT-AC3100 / RT-AC3200 / RT-AC87U / RT-AC87R
Dear RT-AC5300 / RT-AC88U...
