Under the Advanced Settings (should see Wireless). This is where the Wireless Mode settings are located.
However, on the Asus routers, when "Smart Connect" and Mesh modes are being used, this is set to Auto and cannot be configured. (need to disable Smart Connect to adjust). See image below.
However, the 14.40 is usually just a password related error, so changing the Wireless Modes aren't likely to resolve. If you are able to see the network with the Roku, then Wireless Modes are usually configured correctly. The 14.40 error is triggered when password authentication fails. (on wifi6 routers usually because WPA2 is not being used or set to WPA3-only). On older Roku devices, usually because of the special character issue.
You may want to verify that you are not blocking access via the Asus apps and their settings for your Roku device. Another suggestion would be to try disabling "802.11ax / Wi-Fi 6 mode" for troubleshooting under the Wireless settings and see if that helps. (if it does, then indicates there is a compatibility issue between Asus and Roku when router is in ax mode).
Unfortunately Roku and Asus always seems to have issues with each other and updates. It usually is the Asus firmware though (to be fair) but usually takes a bit of time to resolve. Haven't seen additional reports that the latest update is causing an issue. Last big culprit was that when "Protect Management Frames" (PMF or PFM) was enabled, Roku was encountering both password issues as well as internet access problems. This setting is also under the Wireless section.
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I only use Roku and my laptops for streaming on my TVs so can't speak much for the other streaming devices. I know people have very good things to say about the AppleTV as well as the Onn streaming devices (uses GoogleTV).
click image to enlarge.
