Some smart TVs will join open networks if you don't give them one. And I expect that if 5G works as advertised you'll see surveillance capitalism adding 5G connectivity so you no longer have control over connectivity.
E.g. Some TVs will honor wifi off setting. Or alternatively setting the TV to use the Ethernet port.
Or if it needs something on the other end, set up old underclocked Raspberry Pi as a basic router/DHCP server that connects to nothing; power it with TV's USB port.
If you've got a fancy router, connect it to your network with a fixed IP and firewall deny all packets from/to its IP.
If you've got a fancy AP, set up an alternative SSID that connects to an unused VLAN or otherwise routes to nowhere.
(I'm almost astonished that advertising networks haven't switched to using raw IP addresses everywhere.)
The firewall has the same DNS block-lists as the Pi-Hole, but also has subscription lists of IPs to avoid. Most of those are spammers or malware, but can include whatever other category of malfeasance you desire.
With 5G, you will have the same problem. And I'd be very reluctant to buy anything stationary which has 5G connectivity.