am wondering if my WiFi passwords are stored on the device and now could be compromised. Do I need to change my Router password now????
I'm sure they're stored on the device. But, I can't see any risk. Whomever who receives your tv would have to distribute your location and wifi details to someone near you. The chances of that happening are slim. It wouldn't be that valuable to anyone.
You should be connecting your Roku TV to a guest network, one that can't access other devices on the local network. Your router should have options to set that up (it's own wifi name, and password.). But, the big thing is to not be able to access other devices on the network. I think this is important for anything attached. But, it's been reported that Roku is pretty bad with security vulnerabilities. Google about it. I don't think it's as serious as an unprotected device connected to the internet. You'd just be exposed to a hacker in your vicinity (not the entire world). Not allowing connected devices to access each other would limit one devices vulnerability to that device.
"People are often amazed at how much we’ve done with the number of engineers we’ve got." (Roku CEO Anthony Wood, Austin Statesman, Oct 4, 2019). "Amazed" is one way of putting it.