You didn't mention the model # or firmware version of your Roku device(s): Settings/System/About
This is important in general, but especially when new firmware versions are rolling out (OS 11.5).
What is the "Internet download speed" result from Settings/Network/Check Connection?
Did you restart all devices? (Rokus & modem/router/gateway).
FTR, when uninstalling an app, you MUST RESTART the Roku before reinstalling:
1) Remove YTTV/Prime app: Highlight app press Options (*)
2) RESTART Roku: Settings/System/<Power>/System Restart/Restart
3) Reinstall YTTV/Prime app
The YTTV app allows one to manually select the resolution: During playback press Down, Select HD, then Select 1080p or 720p