Monitoring for power cuts in outbuilding

I have an outbuilding with power outlets. There's a local circuit breaker within the outbuilding so power in there can go down independently of the house. I want to set something up to monitor for this and notify me if/when it happens. The building does not have ethernet and is out of range of my home WiFi. I have a spare powerline adapter that I'm not using, so my first thought is to use powerline to get an ethernet socket in the outbuilding and run a simple script on a server in my house to ping a networked attached device that I place in the outbuilding. If there's no response it will send me a phone notification (via a service like Pushbullet) prompting me to check on the power in the outbuilding. This leads me to my main question: I need something to ping which can connect to network via ethernet (my powerline adapter does not have WiFi so I can't use a WiFi smart plug for example). Given its sole purpose will be to act as a target for my ping, I'm wondering what's the cheapest thing I can buy for this purpose?

Also, another question: I'm wondering what alternative approach I could take which does not involve ethernet, powerline or WiFi. I'm thinking I could put a microcontroller in the outbuiling and periodically send a non-WiFi wireless transmission to the house. If it fails to arrive, the power is probably out. The outbuilding is about 15-20 metres from the house. It's a wooden building and the house is brick. Would LoRA be the best choice for wireless transmission here? Given I already have a powerline adapter, I expect this wouldn't be as cheap as the powerline option but using a wireless protocol other than WiFi would be a learning exercise that might also be handy for future projects.