...possibly it only needs a full CAPS replacement.
This page describes a couple of successfull repairs concerning a Zyxel Prestige 660-HW ADSL2 router.
Lightning.After a summer storm I have found my Zyxel 660Hw out of order. It has received a voltage surge from the ADSL line.
Reconstructing PCB lines behind the ADSL socket fixed the problem.
Unstable LAN connection.With age, my Zyxel 660-HW started to be instable on all 4 LAN connections. All other functions were fine.
Symptoms were: poor LAN performances and continually loosing all LAN connections and soon reestablishing them (every 15 seconds or so).
Replacing that cap cured the problem.
Most of the electrolytic CAPs measured bad ESR values. In particular a couple in the power supply (marked in yellow in the previous picture).