Roku devices have no provisions for receiving live over-the-air broadcasts. (Roku TVs with Roku capability built in DO have antenna inputs for this purpose.) So any network programming received on a Roku device must be streamed via one of the streaming channel apps that carry local channels (YouTube TV, fubu, Hulu + Live TV, etc.). It is that channel app that determines which locality's stations you get.
Some (all?) of these channel apps use the location indicated by the IP number assigned to you by your internet provider to determine your location. If your internet provider changes your IP number to one attached to one of its other servers located in a different area, that could change the local stations you are provided.
You can use a website like https://www.iplocation.net/ to see where the IP number assigned by your internet provider indicates you are located. If this doesn't represent your actual area, contact your internet provider and ask to be reassigned an IP in your area.
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