I don't have a magic email/contact number for Lufthansa. It's often worth taking time to scratch around their website. Failing that, either log a complaint or there's merit in using Twitter to get their attention. Keep everything from Lufthansa. Such as your boarding pass, any email stating the lost bag and when it was returned. This will affect the amount you're entitled to.
If folks have an iPhone, the AirTags are the way forward. £25/tag. Replaceable battery. It should last a year. No subscription required. Connect it to your phone.
Job done.
I use mine ALL the time. I have one on my car keys, they're stashed in the nav bag, I have others scattered throughout all my bags. Honestly, best thing ever for travel.
If folks have other phones, consider the Tile. It works in a similar way using bluetooth. I don't know anything about the Tile.