I have 2 Roku ultra already hooked up in my house. I am trying to hook up a third, but the Ulta box is not communicating. I can sync up this remote to the other Roku ultra and it works. I tried to find this Roku ultra box on my the mobile app and nothing. All this one does is sits on the pair remote screen and the white light just random blinks
See if you can get the problem Roku Ultra to connect via ethernet cable. (wired connection). Once you are connected and go through setup and updates, then try again to connect wirelessly. Alternatively, move the problem Ultra to the location of a working Ultra and try to setup connection from that location and TV.
Alternatively, you can try setting up the problem Ultra via mobile hotspot on 2.4ghz preferably, again, if available to you as on option.
If none of above works, Roku has a replacement warranty for devices under a year old.