Is your MLB subscription directly with MLB or is it through Spectrum?
If through Spectrum this is a question you shoud direct to Spectrun re accessing your MLB subscription on their Roku app.
If directly with MLB, you should be able to install the MLB app on your Roku and log into that using your MLB account credentials.
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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Ultra 4800 | Streaming Stick 4K+ 3821 | TCL Roku TV 43S245/C107X