That’s text. This decodes to the character 5, followed by the character 0, followed by another 0.
Maybe Change Timermillis() period on downlink for STM32L0 - #6 by arjanvanb can get you started, but: you should really encode the number in a binary way, and not send ASCII characters.
(And aside, given the data rather than payload_raw, and the lacking dev_id: are you using the deprecated TTN Node-RED library?)