Diagnosis
This is a common issue with Roku TVs, often caused by a combination of factors. While a failing internal WiFi adapter is possible, it's more frequently due to signal interference, a congested WiFi channel, or a software glitch that causes the TV's network stack to freeze. The fact that your other devices are fine points to a problem localized to the TV's hardware or its interaction with your specific network.
How to Fix It
Start with a full network reset on the TV. Go to Settings > System > Power > System restart. After it reboots, go to Settings > Network > Set up connection > Wireless. Re-enter your WiFi password. If the problem persists, log into your router's admin page and change the 2.4GHz band to channel 1, 6, or 11, as these have the least overlap. On the TV, also set a static DNS: in Settings > Network, select your network, then "Advanced settings," and change DNS to manual, entering 8.8.8.8 and 8.8.4.4.
If That Didn't Work
The most reliable solution is to bypass the internal WiFi. Purchase the official Roku Ethernet adapter (model A-3J-1T or compatible for your TV) and connect it to an open port on your router. For a wireless fix, try moving your router at least 3-4 feet away from the TV and any large metal objects. As a last resort, perform a factory reset on the TV (Settings > System > Advanced system settings > Factory reset), which can clear deep software corruption.
Heads Up
The placement of your router is critical. Never position it directly behind the TV cabinet or immediately next to large speakers, soundbars, or surge protectors, as the TV's metal shield and electronics create a significant dead zone for WiFi signals.