Pokéwalker won’t turn on after 15 years – new battery didn’t help. Any ideas?

Can anyone help me with my problem?
After 15 years, I wanted to play HeartGold again with events (thanks, DNS glitch) and the Pokéwalker. Unfortunately, the pokewalker won't turn on.
I've inserted a new battery and also reconnected the cable to the display.
Does anyone have an idea what could be causing this?
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