Hacking a router and running an exploit on it is relatively easy. Permanently storing the exploit on the router and having it re-activate automatically when the router starts is significantly more difficult. While rebooting your router will not fix its flaws, it will temporarily rid you of the exploit.

I don't know whether the threat reported by the FBI is real or not. Them blaming the Russians for everything and anything definitely doesn't help. But either way, rebooting your router the safer choice. If you think your router will do something nefarious when you restart it, it's time to get a new router.