Deutsche Telekom (Magenta / T-Online) runs Germany's largest copper and fibre network. Real-world speeds depend heavily on whether you're on Super-Vectoring (250 Mbps copper) or true Glasfaser (FTTH up to 1 Gbps).
Distance to the DSLAM is everything. Speedport routers show the actual sync rate in the diagnostics page, if it's far below the plan, ask Telekom to check line attenuation.
Speedport Smart 4 is decent, but Wi-Fi 6 mesh kits (FRITZ!Repeater, Eero) handle large flats much better.
MagentaTV uses dedicated IPTV multicast, if your line is unstable, set-top boxes lose stream first. Wired Ethernet to the box is mandatory for 4K.