There is no real "subscription" for a Roku device, there are subscriptions to some channels, but I've never heard of one sending texts, and they would identify themselves as a channel, and anyone asking you to call is probably a scammer. (I'm about 99.7% sure of that last part.)
Hopefully you didn't get scammed into paying money to someone to set up your Roku in the first place. If so, then a scammer like that could continue to mess with you. This is both due to the fact that he could be in control of your Roku account, and because scammers return to already scammed people.
It is a total scam. I fell for this about a year ago when trying to register a new Roku and ending up spending over $130 for the "new" subscription. Luckily when I challenged the guy I was talking towith legal action he agreed to refund the fee to my credit card (which he did). I'm now getting the same text messages - my services will end, first on 12/20, then just tonight I got another text saying my Roku service will end on 12/23/20. Again, I texted "Harry" back threatening legal action if he did not stop texting me and he replied "OK." Bottom line - this is a **bleep**ty little scam that you can ignore.