Smart TV's in many ways are not so smart. There are few things they do that can cause trouble that it is important to be aware of.
1. When you set one up to your SSID & WEP Key, they often also remember the channel the AP is on. If the channel changes, many won't follow and will have to be manually re-setup to learn the new channel.
2. WIFI range extenders can often cause havoc if not properly placed with a consistent strong signal both to the TV and to the router or access point providing the connection.
3. If you have multiple WIFI accesss points with the same SSID (common in larger installs) some TVs will learn the first MAC address they hear and will only associate to it - even if it is not the best signal. On ROKU TV's that learned connection will be very persistent and will require accesing a hidden menu to delete and re-learn. There is a great article on the ROKU hiddem menus that may help:
https://lifehacker.com/all-the-roku-secret-commands-and-menus-in-one-graphic-1779010902