Solved: I'm posting this in case someone else has the same issue (and I've seen similar posts for other streaming services).
After switching to Verizon 5G Home internet, which works great, the local stations on my SlingTV service would sometimes disappear. SlingTV tech support said it was because my IP address was showing in another City & State.
After going in circles a few times checking IP addresses between the 5G router and my 5G phone, I decided to check my SlingTV profile. What I found is that my profile had my billing address (under Billing Information) but not my home service address (under Personal Information). I added my home address under Personal Information and now all my devices (AppleTV, Firestick, PC and iPhone) all correctly see all the local channels.