But it's too expensive...The tennis seems to be the way to go if I've got power and a wireless connection especially with the features it has.I'm open to standalone wireless routers with battery power and connection!

What I do not like AT ALL from the guest's perspective is electronics. 1) batteries/chargers are a pain 2) kids fight over them 3) wi-fi comes and goes 4) passwords!

We reconditioned all of them after taking the 10 unit building over.  

How is the battery life? 

I have about every main tool Dewalt makes as well as 20 batteries.  

We bought ours for $555,000 in September 2019, have installed solar and battery ($30k) and new a/c and furnace (10k) as well as some landscaping upgrades (landscaping and artificial turf).

Squatter deactivated front door smart lock on my unit by disassembling it and removing the battery.

The deal breaker for me is that they don't work on cloudy days or at night and there aren't batteries yet to store enough of the energy, that are acceptable to me.

While I am not usually in favor of solar for a rental this may be a partial solution to your problems with a battery wall. 

I know some video door bells are battery operated so I don't have to do electrical wiring just to install them.