Asus AX53U Slow Performance on Telekom Fiber

Try:

and post a screenshot (the screenshot contains your approximate location and current IP address, so make sure to redact those if you consider these sensitive).

Also please read:

While I think that ~200Mbps is a bit low for WiFi on that device, I believe the AX53U is MIPS based and hence in no way suitable for saturating a 500 Mbps link. (No need to change the router, if you can accept a lower download rate, or you could switch to the cheaper 250/50 tarif once your current contract is over).

Measured throughput of reported link rate? IIRC on WiFi achievable throughput often is in the range of 2/3 of reported link speed...