I bought this on Amazon Canada for winter preheat back in 2018.

Wireless Mobile Cell Phone PDA GSM Remote Control 3 Socket Power Outlet Smart Switch AC 110-220V
Sold by: Top Solid
CDN$ 99.95

I use it with a SpeakOut sim card purchased from the local 7-Eleven. Costs $10 for the sim and a minimum $25 topup per year. I send a text to turn it on and receive a text back showing it is on within a minute. Incoming texts cost .15 each. It has 3 x 120v outlets but is only good for 10A total. I use it for preheating the plane with a ceramic heater on low heat and a sump heater. I haven't had to top it up prior to the end of the year yet. I have used it with both Android and Apple phones on our Bell and Telus networks. Perfect for 3 winters so far.

Best $140 I ever spent

Unit : GSM 4G Auto @ $140 waferstar.com (Shipped direct from China ..Took <10 days for delivery !! Very Surprised at that)

GSM Sim Card: $5/month (Let lapse at any time and reactivate any time)
Speedtalk.com

Unit cell phone programable for engine preheat time 0 - 18 hours (App super simple..so simple to use))

Unit sends txt message when preheat turns on.

Unit sends txt when preheat turns off..

Unit sends txt message when unit looses power (Your preheat tripped a hanger breaker)

Unit sends txt message when power is restored to unit.. (Preheat stays off)

Unit has two programable channels (30A relays !!!)

Antenna remote mounted outside hanger.. (With hanger owners permission )
No signal reception with antenna inside hanger