Sketch: MQTT

First run

1. Push any of the buttons. Led indicator will start blinking and new wifi access point will appear for 5 minutes.
2. Connect to the network from your phone or PC and open URL 10.10.10.1
3. There you will set your wifi credentials, broker credentials and topic + payload for each button. (OPTIONAL: If you use Home Assistant, just use suggested topics.)
4. (Optional) To potentially improve performance and battery life, you can add your router's gateway IP address and also pick a static IP for your remote. You will also have to set the static IP on your router.

Changing settings

Push and hold buttons 1 and 4 until the led indicator starts blinking. To shut down Config AP and resume normal function, just push any button.

Home Assistant

In case you use Home Assistant built-in MQTT broker, you can push and hold buttons 2 + 4 until the led lights up to register the device. It will appear in Settings / Integrations / MQTT: Mosquitto broker. This assumes you have MQTT and Mosquitto correctly set up beforehand.
Use suggested topics and payloads with Home Assistant.