Make sure Wireless Isolation is not enabled. Sometimes firmware updates will reactivate or activate things you don't expect.
Same network, but is one maybe connected to 5ghz and the other not?
I'd try rebooting everything, starting with your router, and then the Rokus..
Ok, this is getting very frustrating. Turns out the router and modem were so old they needed replacing, so Xfinity changed them out for a new single unit. I now can't detect either Roku though the phone app. I've tried watching various Youtube videos to no avail, work-arounds involving renaming your network, etc. I currently live in my boyfriend's parents' house (i.e. someone else's household). Am I screwed/ do I need new Rokus?