The modem is quite expensive, especially if you just want to switch on the heater to preheat the van. I just used one of these,
Smarter Shopping, Better Living! Aliexpress.com
a.aliexpress.com
Ring or send a text and the heater fires up on full blast for 2 hours. Just needs a live and earth and two wires run to the heater. You can't control the heater, only switch on and off, but probably enough for most people.