Actually, I opened the router itself and it was bad caps. I replaced them with some surplus OSCONs and it was good to go for a few more years. Also fixed another Cisco for a friend and it was also bad caps. But those power supplies do often fail.
13.6mF is a lot of capacitance especially for an old G router. My AC1200 Trendnet, in contrast, came with a 1000uF cap on the PA supply rail, which I upgraded to 2200uF along with a few other upgrades like better cooling and high gain antennas.