Not to mention that they can be a bridge onto other devices on the local network. You don't need a PC to infect other kit, and imagine if your fridge encrypted your photos on that NAS and then demanded a ransom. Or took over DHCP duties so it could inject false DNS data, route all outbound traffic through itself to wiretap your net, etc.
Use a VLAN properly and every smart device will think it is the only thing on the network.
For simplicity in writing, say it is a Samsung WiFi enabled refrigerator (as that was what I was considering). Substitute any device manufacturer, it would be the same.
If it can't get to the net, it would simply connect via other access points - your neighbor may be using a Samsung phone or TV, either of them could have been equipped to communicate with other Samsung devices.
Besides other Samsung devices, most ISPs have nation-wide WiFi. Anywhere they have a commercial or residential cable modem installation, that is a spot where the ISP can install an additional (physical or logical) access point to support their nation-wide WiFi. Samsung could be a customer so access points are fairly readily available.
Then of course there is the Amazon Sidewalk the likes that Google/Microsoft will like have. Just ink a deal with Microsoft/Amazon/Google and you can tag on to their nation-wide network paid for by Echo/Alexa/Google-Assist type devices owners. Anyone with an Andriod phone could become a conduit, if not now, may be later.
Amazon Sidewalk (and the likes) is a
case and point how given the means, they will abuse it, eventually. One can hope government would pass a law to prevent such abuse, but then governments often are the abuser as well.
For now, I must take the law in my own hands - I will not give them the means. Come to think of it, I am Rick law - that is I am a Law. So this Law is fighting abuse at least for myself.
Use a VLAN properly...
Glad that fixes it for you. For the other 7b people, though...
And, in my humble way if I am ever humble, hoping to set an example for the other 7 billion people....