I had a pretty shocking moment recently. I bought a cheap simple Chinese 55" TV when I moved into my new place in Hong Kong recently. Worked fine for 6 months. I don't connect it to the internet or cable, it's just a screen in front of a few HDMI devices (Roku + Switch). It recently started randomly playing 30s video ads every time I turn it on. I can't figure out how it's getting them, because I've never connected it to the wifi (nor would I even know how to given the menus are all in Chinese). Does it have its own LTE connection?
Anyways, this has got me pretty annoyed and I'm thinking of tossing it on principle.
It probably has a built in WiFi unity, before tossing it you could open it up and try to find the antennae, they are usually just short bits of wire. Snip those off and see if there is a factory reset option, that should 'cure' it.
Some devices will scan for open wifi networks in the area and connect to those if you don't give them your own network. It's also entirely possible they have deals with ISPs to use the router you rent from them on a separate network (some routers in the US broadcast a 'public' paid wifi network that's separate from yours that anyone can connect to and doesn't count toward your data).
Anyways, this has got me pretty annoyed and I'm thinking of tossing it on principle.