AC68U weak 5 GHz signal

I'm using an AC68U, and inssider Home shows a weaker 5 GHz signal compared to its 2.4 GHz signal. No big deal--I suppose that's normal. I measured the signal on my laptop--it's in another room, same floor of the house, about 50 feet away. The router is about 5 feet off the floor atop my desktop computer with no nearby obstructions. There are no overlapping channels nearby.

Would replacing the router's antennas with longer ones bump up the 5 GHz signal? What are my choices for increasing the...

AC68U weak 5 GHz signal