Roku has no way of knowing you get a subscription through Verizon. Some apps require you to authenticate through your cable provider and others require you to log in with a username and password. The MGM+ app seems to be one of the latter. If you have a username and password that you can use on the MGM+ website, you should be able to use the same credentials in the Roku app. If you don't, you should ask Verizon why you don't.
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