TomatoUSB

I was running TomatoUSB Toastman's mod 7498.1 and it got hacked yesterday (by someone from China apparently). The only external services i was running on it that could have been somehow hijacked were
OpenVPN (2.2.2 version) and Dropbear's ssh (apparently it was 0.54 version). I'm not sure how attacker got root access to the router. I think outdated Dropbear was at fault because apparently there was some security issue before 0.55 according to Dropbear's changelog however i want to hear your opinions. I'm going to update the firmware today so i want to hear your opinions on what might have happened or what should i do to be more secure (like running dropbear on external port)

Don't run external ssh port if you don't know how to secure it.