Quoted:
I'm trying to setup a home theater system / network for my folks. They have no internet at the lake house, but the plan is to have the TV in another room from the PC.
All we want to do is have movies on the PC then stream content via the WiFi (dlna) network to the non-smart TV. Any recommendations on a device to interface between the TV and the WiFi?
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Huh? I think you have made this overly complex.
Without internet access, there is nothing to "stream".
Get a Bluray player and connect it to the TV for your movies.
If you want to interconnect their PC's into a home network, get a network hub. Their PC's can connect to each other through this hub to create their home network even without internet access. The interconnections can be either wired or wireless.
IMO, wired connections are much more reliable but wireless is still probably the way to go.