Bkself
On the working Roku Ultra, you can go to Settings/Network/About that will tell you which band you are using. (ch. 1-11 indicate 2.4 ghz band 802.11 b/g/n and ch. 36-165 indicate the 5ghz band 802.11 n/ac).
The Ultra devices are dual band and can connect to either the 2.4ghz or 5ghz band of your mobile hotspot.
On the non-working Ultra, since you factory reset it and it won't connect to the network, you probably won't be able to look at the signal strength from the Wireless Hidden Screen. For some reason, Roku removed the graphical antenna indicators which were helpful in examining if the antenna hardware was working. (Home 5x, up, down, up, down, up).
Sounds like you may just have a failed/defective device though.