Do you have just the single smartphone? No tablet? If you have another mobile device, you could add the app to that device.
You didn't say which phone you have, nor the OS, but there have been reports of issues with some iOS devices using iOS 12.5 (current is 15.5). As Roku app claims to support iOS 11 and greater, this issue should not be happening, but it is, at least for some people.
Might this be what's going on with you?
Also, always have a physical remote. If you device ever loses connection to the network, you must have a remote to reconnect it.
Roku Community Streaming Expert
Note: I am not a Roku employee.
If this post solves your problem please help others find this answer and click "Accept as Solution."