What model and model number is your Roku? Settings > System > About will show you, or if you can't access the Roku home screen, look for a label on the device.
Some lower end Roku models only support the 2.4 GHz wifi band. If your mobile hotspot is set to use the 5 GHz band, try setting it to 2.4 GHz.
Roku Community Streaming Expert
I am not a Roku employee, only a user like you. Please, no support questions via private message -- post them publicly to the Community where others may benefit as well.
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Streaming Stick 4K+ 3821 | Streaming Stick+ 3810 | TCL Roku TV 43S245/C107X