A remote control receiver with a much beefier relay than normal. I'm not sure I'd actually try using it anywhere near 30A, but the bigger relay would make it better suited to higher loads than the units with tiny relays.

As with many of these units, the antenna wire should be regarded as being live at full mains voltage and located and treated accordingly.

The circuitry is very typical of these units, with a nifty buck regulator and classic receiver module that means the unit has a standby power of just quarter of a watt.

Here's how to program this unit and many others that use the same software.

To clear all existing codes, press and hold the button for about 5 seconds until the LED starts flashing, and then release the button and wait for a few seconds as it erases existing codes.
If the LED doesn't start flashing then the unit may use a different erasing technique. Turn it off and on again and then click the button 8 times. That should clear the existing codes.

To select a mode press the button between 1 and 7 times, observing the brief LED flash with each press. After the required mode has been selected the LED will light to show it's waiting to receive a signal from the chosen remote button.
Press the remote's button and the LED will go out, or if it's the two button toggle mode it will blink and then wait for the second button to be pressed (which doesn't need to be on the same remote.)

If desired you can pair several other remotes by repeating the procedure. They can all have different functions.

Modes:-
1 - Single button momentary action. Non latching.
2 - Single button toggle on/off.
3 - Two button on/off. Each button is assigned independently.
4 - Single button timer 5-second delay.
5 - Single button timer 10-second delay.
6 - Single button timer 15-second delay.
7 - Single button timer 20-second delay.

Some similar units have an adjustable link for setting the mode. It usually offers momentary, latching or two button toggle. The button may only be for learning a code.

These units are common on eBay, usually supplied with a remote switch or key fob with a frequency choice of 433MHz or 315MHz. In the UK 433MHz is a standard frequency.
The transmitters are fairly standard and many different units with the matching frequency will work.