DIAGNOSIS Faulty WiFi adapter on the main board or interference from the TV's own components.
FIX Change the WiFi channel on your router. Use channels 1, 6, or 11 on 2.4GHz. In the TV's network settings, set up a private DNS. Use 8.8.8.8 (Google). This often stabilizes the connection.
IF THAT DIDN'T WORK Use an Ethernet adapter. Get the official Roku Ethernet adapter for your model and plug directly into your router. This bypasses the internal WiFi hardware entirely.
HEADS UP Do not place routers or soundbars directly behind the TV. The metal shield and electronics cause severe signal blockage.