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TL;DR: Make sure you run over HTTPS when you are working with Service-Workers.
Ok, so… apparently I completley forgot, that service-workers work ONLY over HTTPS.
Previously I was testing FCM on my PC, and I added an exception to
chrome://flags/#unsafely-treat-insecure-origin-as-secure
but I didn’t do so on my smartphone, and that’s why it didn’t work.
Source:stackexchange.com