He used smart home technology, sensors, digital door locks, an online thermostat, and outside cameras so he could keep an eye on the comings and goings of his guests.

You have to weigh the risk/benefit ratio and to me there is not enough risk for me to provide network separation between the guest using internet and my wifi thermostat, ring camera, streaming TVs.  

House Thermostat - The thermostat is not to be adjusted by any tenants.

It's a steel tank with a set of elements and thermostats.

I have been using Nest Yale lock, and thermostat.  

I have wifi thermostats, wifi water heater, outdoor security cameras, but still use a lock box.

Then I installed a new thermostat in the apt., tripped over their still shivering cat & staggered home.

These may be signs that there are other maintenance issues that were not taken care of properly or at all.Check the windows for gaps as well.Cracks in the interior walls or ceiling.Turn on the faucets to make sure there is good water pressure.Test the thermostat to make sure the heat kicks on.Hope that helps.

I have Internet in my student rentals that i provide as @Cooper Bert mentions, as matter fact i have the door locks on one of them and thermostats on all of them, this way i can control the heat from anywhere.

It’s an older unit and I was planing on replacing in 2 to 3 years but the technician told me the reason the unit went out was because the tenants left the thermostat on at 45 degrees for several weeks, meaning the AC wasn’t able to turn off and reset.