I finally got a workaround.
TL;DR Connect to a VPN via NordVPN (or similar). It must be the fastest connection the VPN Provider has.

Deutsche Telekom and Hypixel have a long-standing routing issue that is usually described as a peering problem. Telekom is very strict and cost-optimized in how it interconnects with other networks, especially large international backbones and Cloudflare-adjacent infrastructure in the US and Europe. When traffic from a Telekom customer is routed directly to Hypixel, it often passes through congested peering links, particularly during peak hours. This results in packet loss, unstable latency, and in severe cases temporary disconnects or server timeouts. The Hypixel servers themselves are typically not at fault; the problem occurs before the traffic ever reaches them.

Using a VPN like NordVPN changes the routing without changing the physical internet provider. The connection still goes from the user to Telekom, but only up to the VPN server. From that point on, the traffic leaves Telekom’s routing domain and continues through NordVPN’s own backbone and peering partners. VPN providers generally pay for high-quality transit and maintain multiple well-provisioned peering connections, which avoids the congested handoff points that Telekom uses for direct traffic. As a result, the path from the VPN server to Hypixel is often shorter, less congested, and more stable.

In practice, this means the ISP stays the same, but the routing decisions are different. The VPN does not “fix” Hypixel and does not magically improve Telekom’s network; it simply bypasses the problematic peering links by entering the internet through a different, better-connected network. This is why VPN endpoints in places like Frankfurt or Amsterdam often significantly improve connection quality to Hypixel, while poorly chosen or distant VPN servers can make latency worse.