I've found that when I've encountered wifi problems with my Rokus I've normally been able to solve them by restarting the router. After the router comes back up, restart the Roku via Settings > System > System restart (for a Roku TV it's Settings > System > Power > System restart) to start with a clean slate and try to reconnect.
This makes no sense to me and I don't understand why it would work, especially since other wifi devices in the house find the network and connect without issue, but it has worked for me on several occasions.
- Roku Streaming Stick +, 3810X, Ser YH0059427035, wifi - Samsung UN55ES6100
- Roku Streaming Stick, 3600X, Ser 5S56D8240827, wifi - JVC EM37T
- TCL Roku TV 43S425/C107X, Ser X000001R60KV, wifi