DIAGNOSIS Yes, it's normal for these units to run quite warm. They're powerful tri-band routers packed into a small, sealed enclosure with no active fan. The top is a heat sink.
FIX The fix is about ensuring proper ventilation. 1) Make sure the unit is standing upright, not lying on its side. 2) Leave at least 2-3 inches of clear space on all sides, especially the top and back. 3) Don't place it in an enclosed cabinet, on top of other hot electronics, or in direct sunlight. Putting it on a stand or even small rubber feet to elevate the bottom a bit can help airflow.
IF THAT DIDN'T WORK If it feels *hot* enough that you can't keep your hand on it comfortably, check the power adapter. Make sure you're using the original one. A failing or incorrect adapter can cause excess heat. Also, during heavy, sustained data transfers (like large backups), they will heat up more.
HEADS UP The warmth is by design. The casing is meant to dissipate heat. However, excessive heat over long periods can shorten the electronics' lifespan. Good placement is the key to managing it. It's not typically a fire hazard, but like any electrical device, don't smother it.