I'm not aware of any systems that work without wifi as you'd have to have some kind of cellular reporting system like a SimpliSafe or similar. 

They do make a few cameras that work off of the cellular network as opposed to a wifi - I think they are fairly affordable as well.

During renovation: getting a wireless solar cellular camera with motion sensors, night vision and lighting or post Renovation a Ring and or simplisafe system while your project is on the market.

And if you are subscribed to a monitoring plan that includes cellular backup, your Base Station will still communicate with our cellular connection back-up."

I tried a lot of the free resources and most of the numbers are not working.Look up “National Cellular Directory”, they might have what you seek. 

After closing sold a cellular tower lease that generated $790,000 in cash reducing our acquisition price down to ~$56.00 per SF What made you interested in investing in this type of deal?

We use a product called Vital Oxide in our flips to neutralize odor and disinfect.

In the event that you do have a true squatter its a fairly standard eviction process which is unfortunate but does happen in this business.On a preventative note if you have a vacant unit you can put in a security system that uses cellular and you will get notification if anyone comes in. 

I am changing out the older cellular shades in our place with standard 2" Levelor blinds from Lowe's.

Get a spiral Binder and write down everything in that binder as well as take pictures with your cellular phone.