I personally have the following that I am very happy with;
- Aotec Range Extender 6 - There is a newer 7 series I believe [Edit: Note this link is for a 2-pack]
- Ring's Range Extender Gen 1 - There is also a new Gen 2
The Ring devices can be paired directly with HE using this community driver. Both generations have battery backups, so they will continue to work without power AND send that state to HE. I have a rule setup that will tell me if the power goes out as I keep my HE behind a UPS.
