Did you configure a action in your esphome yaml when the button is pressed for 5 seconds then? If not you can be 110% sure there is no esphome running (yet) on your device…
It looks like there is a bunch of different sonoff mini’s out there (various hardware and soft/firmware revisions) which are flashed differently with ota.
@meneelyt gave a nice write up here: How To: Flash a Sonoff Mini to ESPHome – 2021-01 version
If you can’t access it in any way by ota you are forced to use the serial method (kind of tricky with this device because it only has tiny solder pads)…