If one ISP is not willing to announce your prefixes, then find another ISP that will agree to do that. (Or find the right person at the ISP to talk to; if you can afford a whole /24 then you probably can find a business contact.)
But if your direct ISP won't do BGP, it's not entirely uncommon to find a remote ISP which does – that is, announce the prefix at a nearby IX or datacenter, then tunnel everything to your location using GRE (or similar). Sometimes this involves setting up your own router at that remote location, sometimes you can run BGP directly from your location across the tunnel. (You will still access it through your local ISP, of course, but it doesn't need to offer anything special besides a static IP.)
It shouldn't be hard to find a commercial provider (carrier) who offers this service in Germany.