You can add a fan, as all WRT AC Series routers after the 1900AC v1 retained PWM fan support, however depending on the current draw of the fan, you may need to up the power adapter to 4A output.
- Noctua makes really quiet small diameter fans, and I utilize one of their 40mm fans on an 8C 2.4GHz Atom CPU that wasn't receiving enough CFMs of airflow to be passively cooled.
I would try the patch in @hnyman's post I linked to below before adding a fan.
It's not normal and if it was running at 75C before the overclock, it's likely either because of the lack of an upstream driver in Linux (which @hnyman talks about in his thread linked to below), and/or it's in a location with inadaequate CFM airflow.
- Passively cooled ARM and ATOM CPUs require a minimum CFM airflow of ~2.5cfm - 3cfm for adequate passive cooling, and IIRC, the normal operating temp should be in he 40C - 55C range.
See this thread by @hnyman. It's for the WRT3200ACM, but all WRT AC Series routers, except for the 1900AC v1, utilize the Armada 385.