Check your wireless router for an option like device isolation or client isolation on the SSID you're using. If Alexa and Roku are on different SSID's (I'm guessing they aren't) then something like network isolation or AP isolation would prevent them from seeing one another. Just being on a guest network might prevent it by default.
Anonymous Would it matter if one is on 5GHz and the other is on 2.4GHz? I can't tell which of those either is on.
FWIW, I figured out how to check, and both Alexa and Roku ate on 5MHz
It doesn't make a difference if AP or Network Isolation is off. It might if it's on.
It definitely won't work if Client or Device Isolation is on.
Router manufacturers can call these options whatever they like, so look around.
Guest networks typically prevent clients from seeing one another, and you may or may not be able to disable that. It's up to the manufacturer.
Some people complain when they try to use their smartphone app as a remote control and it can't find the Roku. You could try the app, and if it works to control the Roku, this isn't your problem.
Anonymous My Roku Remote app works on my Android device.
I'm def not on guest network.
"Would it matter if one is on 5GHz and the other is on 2.4GHz? I can't tell which of those either is on."
My recollection is Alexa is 2.4 GHz only. Looks like you have a dual-band Roku. You could disable your 5 GHz radio to keep them both on the 2.4 to check that.
I'm assuming you have same SSID using both radios, NOT something like SSID and SSID-5GHz.
